What would a be a good fan for a Kraken x41 liquid cooler or just a really good radiator fan.

The Noctua fans would work just fine but the fan that comes with it is not bad. The Notcua's work well for a radiator and use the newest 140mm blade design from Noctua.

If you mount the x41 on the rear of your case. some SP140's will work as well. they use a sleeve bearing so I would mount the horizontally (PS and yes I mean the SP140, no one on the internet but me seems to know that they are for sale).
 


The X41 uses 140mm fans, not 120mm.