Noctua's nh d14 is widely regarded as 'the best' at the moment, obviously that depends on it fitting in your case, and being installed correctly
not to up on best thermal paste, arctic pro amongst others is good
And your case doesn't already have a mesh for a 120 at the back? what case is it?
check some reviews out to get an idea of its capabilities,
but with 2x120's in the back anyway, I'm not sure adding another is going to do anything except increase noise levels hehe
try googling **Your current cooler vs Noctua DH14** see if anyones actually set them head to head