What makes a good AIO?

[J.Munslow]

I'm looking to replace my aging H55 and wondering what I should be looking for in an aio? 

My current assumption is that the pumps and radiators perform basically the same regardless of manufacturer (some differences in aesthetics of course) and that the performance difference really comes from the fans? 

 

Is that the case? I'm looking for a 280 or 240 aio and have noctua fans available to strap onto the radiator so should I just go for whichever brand is on sale this week? 

[RONOTHAN##]

3 minutes ago, J.Munslow said:

My current assumption is that the pumps and radiators perform basically the same regardless of manufacturer

Not exactly. Most are just rebrands of the Asetek design, those will all perform more or less identically, but there are other manufacturers like Arctic and EK that have redesigned the pumps and do outperform the standard Asetek design. In contrast, there are also companies like MSI that redesigned the pump and it performs worse than the Asetek design. 

 

5 minutes ago, J.Munslow said:

I'm looking for a 280 or 240 aio and have noctua fans available to strap onto the radiator

Get a 280mm AIO if it's available, it's closer in performance to a 360mm AIO than it is to a 240mm AIO, despite what the size figures would lead you to believe.

 

6 minutes ago, J.Munslow said:

should I just go for whichever brand is on sale this week?

For the most part, prioritizing getting it from one of the other brands like Artic or EKWB. 

[filpo]

2 minutes ago, J.Munslow said:

I'm looking to replace my aging H55 and wondering what I should be looking for in an aio? 

My current assumption is that the pumps and radiators perform basically the same regardless of manufacturer (some differences in aesthetics of course) and that the performance difference really comes from the fans? 

 

Is that the case? I'm looking for a 280 or 240 aio and have noctua fans available to strap onto the radiator so should I just go for whichever brand is on sale this week? 

this post shouldn't be in custom loop and exotic cooling.
A good AIO would have a quality pump, no leakages after years, optimally its warranty length. Good enough fans, jsut have to be high airflow, don't need to be quiet (like the fans on arctic AIOS aren't the quietest, but are very good for cooling and thick rad means very optimal cooling) and a good maybe thin maybe thick rad. 

For you if your case has enough clearance for a thick radiator and the noctua fans on top of that (not literally) then go with something like the arctic freezer liquid 240 or 280 where you can replace the fans (depending on which noctua fans you have, you choose between the sizes) and if your building in a small space (such as matx or itx case) then check out the h100i rgb elite 

[PDifolco]

Back to OP questions, what makes a good AIO is

 

Rad size/width and overall density of fins, plus the metal from which they're made (mostly all are aluminium, but copper is better)

Block design, smoothness, fin stack and metal used

Pump efficiency, as @RONOTHAN##pointed out

Fan size, efficiency and pressure, obviously the more, faster and bigger fans you have the better, but fan design is important too (Noctua making the overall best fans, Phanteks T30 are exceptional as well)