I assume you are talking about AIOs. And as to that I'd answer picking the best is easy cause none of the others make sense.
1. 120mm / 144mm AIOs .... these cost $50 and up and 95% of them are outperformed by the $20 Hyper 212. The rest are outperformed by any of the moderately priced air coolers and they are way quieter.
So I see no "Raison d'être" for 120/140mm AIOs ... air coolers kick their tail in cost, performance and noise.
2. 240 / 280mm AIOs .... these go up to $115 and more and fall into 2 categories:
a) The ones that maintain moderate fan speeds are bearable... for example the H110 ($115). It matches the Noctua DH-14 in performance ($75) and almost catches the $60 PH-TC14-PE and they are within 1dbA of one another. These I can see to an extent, but you are paying twice as much for the priviledge of telling ya friends ya use water cooling. That's all ya really get out of it.
So again, I see no "Raison d'être" for 240/280mm moderate speed AIOs ... moderately priced air coolers do just as well or better thermally and acoustically and kick their tail in cost.
b) The other category is the extreme speed 240/280s exemplified by by the H100i. I wanna leave the room when these things run w/ 55% or more CPU load. You can listen what that sounds like here:
http://martinsliquidlab.org/2013/03/12/swiftech-h220-vs-corsair-h100i-noise-testing/
Many peeps buy these things and being unable to stand the noise from the 2700 rpm fans and replace the fans with 1200 rpm models. But as you can see here, the cut in fan speed results in a drastic drop in cooling
http://martinsliquidlab.org/2012/04/15/alphacool-nexxxos-xt45-360-radiator/4/
The drop from 2700 rpm to 1200 is comparable to what we see above going from 2200 to 1000 and as we see performance drops by half. And while a better brand and better quality fan will affect performance there is no "magic' solution to compensate for the huge > 50% drop in fan speeds. The H100i w/ 1200 rpm fans ($130 investment) now performs like a $20 Hyper 212.
Even at these extreme speeds, the H100i is outperformed across the board by the better air coolers like the Cryorig R1 and the Noctua DH-15.
So, with more noise and less cooling, I again see no "Raison d'être" for 240/280mm extreme speed AIOs
3. Finally this side of custom water cooling, the only WC AIO's I can recommend are the Swiftech H220-X and H240-X.
a) The kick tail in performance, soundly thrashing everything else out there. At comparable noise levels, the Swiftech H220-X gets you 2C at the cost of 2 dbA versus the better air coolers.... the 240 version is 4 dbA quieter tho and also produces better thermals. b) Both the air coolers and the Swiftechs thrash the extreme speed AIOs and do it at between 1/4 and 1/8th the fan nosie.
b) The Swiftechs are OLC's not CLCs meaning the loops are open not closed. With all the Asetek based units, it's a sealed system. The loops on the Swiftechs however can be opened up and water blocks from MoBo, RAm and GPUs added.... no need for another pump.
c) The Swiftech's are all copper / brass.... no possible issues of galvanic corrosion as we see when mixing aluminum radiators and copper cold pates as we find on all the Asetec based units.
d) The Swiftechs have a resrvoirs and visual coolant monitor.
e) The pump power is easily strong enough to handle the addition of 2 GFX cards, and another radiator allowing you to add water cooling to MoBo and 2 GFX cards down the line.
I could go on but better said here:
http://www.hitechlegion.com/reviews/cooling/liquid/40870-swiftech-h220-x-open-loop-240mm-cpu-cooler-review?showall=&start=3
.....Many looking to delve into the world of liquid cooling start out with a unit that is ready-to-go right out of the box, simply because the idea of setting up a loop can be a little daunting. However, with a CLC you are limited to only what it is, and only the performance it can deliver as assembled. With that in mind, the H220X makes infinitely more sense as a starting point into liquid cooling. It is ready to go out of the box and requires no maintenance, but it is also fully expandable when you are ready to take the next step. Even if you aren’t looking to take the next step, the H220X outperforms every CLC on the market, and does it at more than 20 dB quieter. ....
.....The MCP30 pump has a head pressure of over 2 meters, with over 1GPM flow and PWM control, making it a perfect choice for a 3-4 component loop. The radiator is an excellent quality copper/brass piece that delivers excellent low rpm performance, and the attached reservoir is easily accessible. 3/8” ID tubing is used throughout, and there is plenty of length to simply make one cut to add a GPU in most builds. In other words, you have a small loop in a box with every component being top quality, and may be the same components you would have been looking at in the first place.
Installation of the H220X was very easy, probably the easiest liquid cooler I have ever done. A huge part of this is thanks to the use of the Apogee XL block, which is a flagship top tier block and has a mounting system fitting of that stature. I had noted just how easy the install was when I reviewed the block alone, and comparing it to the Asetek/CoolIt/etc. kits, it is absolutely a world better. T....
In terms of performance, well….we could simply leave it at the fact that the H220X is simply the best performing out-of-the-box cooler you can buy today. Period. It slightly betters its predecessor, the H220, as well as the Glacer 240L that is equipped with far more powerful and louder fans. The NZXT X60/61 comes close in terms of performance, but at the expense of far more noise and far less compatibility. 240mm CLCs can’t touch the H220X in all out performance, and at tolerable noise levels the H220X flat out embarrasses them. The Cryorig R1 and Noctua NH-D15 come closest in matching the H220X in terms of performance and noise, but fall short. ....
.....To sum it up; the H220X offers better performance, lower noise, better aesthetics, flawless design and build, better components and the option of expandability when compared to a CLC. Putting it gently, choosing any CLC over the H220X would be doing yourself a huge disservice. If you are looking to step into liquid cooling, or looking to start a small loop, the H220X is the obvious choice and comes with our highest recommendation and Hi Tech Legion Editor’s Choice Award.
Simply put .....
If ya have $150 to spend, the H240-X is unequaled and provides long term expandability.
If ya have $140 to spend, the H2240-X is surpassed on;y by its Big Bro
At $100 - $140, nothing make sense.... the $90 Cryorig beats everything on the market.
At $70-$80, nothing tops the $60 Phanteks PH-TC-14.
At $20-$50, there's some interesting coolers, but with the demise of the 612 and the lack of availability of the Mugen's on this side of the pond, i can't recommend anything but the Hyper 212.
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