80mm or bigger dual ball bearing or the rifle bearing ones would be good.
To me, fans are fans....only difference is the noise. Now that is for case fans - you can get technical about 120mm moving more air at lower rpm..blah blah...you have a case that has 3 or more fans in it - you are fine, no matter what the size of them are
I usually get whatever Newegg has for the volume discount of buying 10 at a shot - had some thermaltake, enermax, coolermaster....all dual ball bearing, all worked the same except for that lousy .0000001 difference between the enermax and the thermaltake..oh how that bothered me
am I too synical? :p