But the over-riding question is which is the best? Where should the smart money go? Which of these three is the phone you want in your pocket today and over the next year?
You've asked and we've delivered. Having lived with all three flagship devices, here's our take on how they stack up.
The HTC One (M8) features a 5-inch display with a 1920 x 1080-pixel resolution, a pixel density of 441ppi. The Samsung Galaxy S5 has a 5.1-inch Super AMOLED 1920 x 1080-pixel resolution and 430ppi. The Sony Xperia Z2 has a 5.2-inch display, again with a resolution of 1920 x 1080 pixels, this time coming in with a ppi of 423ppi.
In reality there's little discernable difference in the detail or the sharpness of these displays, each is excellent, but they all offer a different experience.
Samsung's Super AMOLED display offers more vibrant colours and deeper blacks than the HTC or Sony, although the Z2's Triluminos panel boosts colours with X-Reality and Live Colour LED. Both offer more punch than the HTC One (M8), but both can easily slip into looking unrealistic.
The HTC One (M8) is more realistic in its presentation of colour. Which is "better" comes very much down to personal preference: Samsung has plenty of pop, Sony lets you alter the colour temperature of the display and you can turn X-Reality off for more realistic results; HTC sits in the middle ground as a good all-rounder.
There are other minor details that make a difference: the Samsung Galaxy S5 gives you slightly more screen space because the HTC and Sony have on-screen controls.
READ: Samsung Galaxy S5 review
Although Sony and HTC feel more Androidy, following the lead of the Nexus devices with on-screen controls, in most situations, the Samsung gives you more space to fill with content.
All the displays offer great viewing angles, plenty of brightness and a wonderful array of colours. The Samsung copes the best in bright conditions and offers very easy access to controls, so if the display is the most important factor, it might be the SGS5 that draws your eye.
Each of these devices is powered by the Qualcomm Snapdragon 801 quad-core chipset. Both the HTC One (M8) and the Sony Xperia Z2 are clocked at 2.3GHz, whilst the SGS5 is clocked at 2.5GHz, so it's notionally faster on paper. (There's an Asian M8 also clocked at 2.5GHz.)
The One M8 and SGS5 both have 2GB of RAM, Sony has a trump card with 3GB and in heavy application loading, like big games, it is a touch faster than the other devices.
However, at a basic level, it's the HTC One (M8) that feels fastest. Although each device offers very similar hardware, HTC's Sense 6.0 UI feels the fastest in daily tasks like unlocking, opening folders and so on. The SGS5 is noticeably slower in this regard.
Sony sits in the middle offering plenty of snap and as we said, it's a touch faster on those intensive app tasks.
The lag we've found in the SGS5 seems to be down to the latest TouchWiz software and could well be deliberate. A small tweak could see it offering more snap and make the device better overall. Sony and HTC on the other hand, feel like they have the edge as they currently stand.
The HTC One (M8) features 16GB of on-board storage, a microSD card slot capable of up to 128GB, and 50GB of free additional Google Drive space.
The Samsung Galaxy S5 features 16GB and 32GB on-board storage variants, with microSD support for cards up to 128GB, and 50GB of free cloud storage through Dropdox for 2 years.
The Sony Xperia Z2 comes with 16GB of internal storage and a microSD slot that will handle up to 64GB.
There's little to call between them: the big difference here is that HTC has stepped up its game in offering microSD support on this level of device. This has long been an advantage offered by Samsung, but the playing field has been leveled, although that larger storage capacity offered on the SGS5 has some appeal.
The HTC One (M8) features a 2600mAh battery that is supposedly 40 per cent more efficient than the first HTC One. We've found performance very good, often lasting over 24 hours with ease.
The Samsung Galaxy S5 features a 2800mAh battery that will get you through the day, although typically we've not found to last quite as long as the HTC One (M8).
The Sony Xperia Z2 has a 3,200mAh battery, topping its rivals for pure capacity. It will also get you through the day in normal use, so whichever you choose, you'll have much better battery life than older handsets: the days of running out by lunchtime are behind us, unless you're playing intensive games or making back-to-back calls.
READ: HTC One (M8) review
All three also offer power saving options. The Xperia Z2 has the best granular control for savings in normal use through its Stamina Mode, letting you configure which app can draw data in the background.
The M8 and the SGS5 both have an extreme power saving mode that reverts the phone to a dumb state of basically text and calls only, so it will last for much longer. This is really an emergency state designed to get you home to a charger.
However, on the Samsung you can pull the back off the phone and swap the battery, the only device that offers that option.
The HTC One (M8) features the same UltraPixel sensor (f/2.0 aperture) found in the original HTC One, although it's a better all-round performer than before, supported by a good twin tone flash.
It also has the Duo Camera for depth perception only, meaning there's a range of fancy features on offer, and 5-megapixel (f/2.0 aperture) front-facing camera which is excellent for selfies.
The Samsung Galaxy S5 features a 16-megapixel sensor with fast 0.3 second capture speed. The smartphone can also capture Ultra HD video - something the HTC One is incapable of - and there are a range of smart functions, including "selective focus" which produces similar, if slightly inferior, results to HTC's Ufocus without the additional lens.
The Sony Xperia Z2 on the other hand has a 20.7-megapixel snapper on the rear, with a range of clever features, also including 4K video. The Xperia Z2 rarely uses all that resolution, but even at the default 8-megapixels, returns some very nice results in most conditions, with plenty of control options, and SteadyShot digital stabilisation which is effective.
Whilst the HTC offers some great results and nice features like highlight videos, if it's straight-shooting quality you're after, with the option to crop into those photos, in most conditions the Sony and Samsung win out, but HTC's camera performs well in good conditions and is very fast.
Software and user interface
The HTC One (M8) features Android 4.4 KitKat and Sense 6.0. The Samsung Galaxy S5 features Android 4.4 KitKat and a new version of TouchWiz. The Sony Xperia Z2 features Android 4.4 KitKat with the Sony interface over the top.
All these user interfaces offer bags of features. If this was simply a case of comparing who offers the most features, then Samsung would win.
However, there's so much in the SGS5 that a lot seems superfluous. What is good, though, is Samsung's sensible use of display space: there are features that let you multitask better on a big display. Sony offers some mini apps, but is nowhere near as comprehensive as Samsung's offering.
Sony has added the usual customisation to the Xperia Z2, adding plenty of Sony features in the process. There's a big push on Sony's movies, music and gaming content, with the addition of the What's New portal in the Google Now shortcut. There's a fair amount of bloat too, with lots of Sony apps added.
The HTC One (M8) offers Sense 6.0, which is visually refined over previous HTC devices and looks and feels more mature than Sony and Samsung's offering: it feels like it's been updated the most out of the three and we think it feels the fastest in daily operation.
All three are well connected, with integrated social contacts, access to networked media and so on. We think HTC Sense offers the most refined experience, but admittedly this comes down to personal preference.
Design and build quality
The HTC One (M8) measures 146.36 x 70.6 x 9.35mm and weighs 160g, the Sony Xperia Z2 measures 146.8 x 73.3 x 8.2mm and hits the scales at 163g. The Samsung Galaxy S5 measures 142 x 72.5 x 8.1mm and weighs 145g.
But these numbers only tell a tiny part of the design story. Both the HTC One (M8) and the Sony Xperia Z2 offer a solid, premium build, while the SGS5 sticks to plastics, but the design of all is very different.
The HTC One (M8) features a machined aluminium body that wraps around the sides making for a 90 per cent metal finish. It's a triumph of design, it looks stunning and feels sensational to hold, fitting neatly into your hand.
READ: Sony Xperia Z2 review
The Sony Xperia Z2 is also a great looking device and comes with a tempered glass finish with an aluminium trim for a flat monolithic effect. However, the flat design makes the Xperia Z2 more uncomfortable to hold and less practical in daily use than the similarly-sized HTC device.
The HTC also seems to make better use of space with the front-facing BoomSound speakers filling the space that's mostly empty on the Xperia Z2.
The Samsung Galaxy S5 is mainly plastic, sticking to familiar design, but with a more tactile back for better grip. It's the lightest and most compact of the trio, but arguably has the cheapest finish.
Both the Samsung and the Sony, however, offer water-resistance. The Sony is the best protected, but both will survive use in the rain, or being dropped in the toilet.
However the HTC One (M8) is the most premium of these devices. Unless you're particularly prone to losing your phone to water damage - or specifically want to take photos in water - the HTC One (M8) is the phone to lust after from a design point of view.
Both the HTC One (M8) and the Sony Xperia Z2 offer front-facing speakers. For the HTC One, that's the tried and tested BoomSound speakers, but they are louder than before on the M8.
The performance is class-leading, far better than those on the Xperia Z2. If you like ditching the headphones, the HTC One (M8) is the device for you. The Samsung external speaker is rather poor compared to its rivals.
However, Sony has another audio trick. It has on-board noise cancellation, so when you connect the Sony MDR-NC31EM headphones, you'll get great noise cancellation without needing bulky headphones. In the UK those headphones will come in the box, which is a really good extra.
Headphones or speakers? Both the HTC and Sony devices offer something special on the audio front.
Additional hardware features
While Samsung might not be fighting in the audio arena in the same way as Sony and HTC, it offers some unique features of its own. There's the fingerprint scanner on the front of the phone and the heart rate sensor on the back.
The heart rate sensor ties into Samsung's renewed focus on health and fitness, although may quickly be dismissed as rather gimmicky. The fingerprint scanner has attracted some criticism, although we had no problem getting it to work.
Sony's uniqueness mostly comes down to content, with Sony pushing its music and movie services hard.
HTC's uniqueness comes from the Duo Camera and the clever Zoe videos. Although some of the features, like background defocus, are recreated by the Sony and Samsung, the HTC is generally faster, with better results and you don't need to choose a separate camera mode, you just point and shoot.
The specs of these three devices are very closely matched. They all offer a large, sharp displays, plenty of power, expandable storage, great cameras and battery life that will get you through the day.
Pick any of these phones and you'll be packing one of the best Android devices on the market. Importantly, however, you have choice: choice in design and functionality, but will have to pay for a top fight device: the HTC One (M8) is £559.99 (SIM free), the Samsung Galaxy S5 is £579 (SIM free) and the Sony Xperia Z2 is £599 (SIM free).
But for us there can only be one winner. The HTC One (M8) is the best smartphone in its class, down the premium design, the powerful hardware and a day-to-day user experience that's slicker and faster, standing it aside from its rivals.
You may not share the same opinion and the pros of the SGS5 or Z2 might win you over, so be sure to tell us which you'd choose in the comments below. You can find lots more information in our detailed reviews of each device.