I guess it depends on what your goal is - a lot of cooling or quiet cooling or something else. I tend to go for quiet, so YMMV. I've had good luck with
these Enermax speed adjustable 8cm fans from Newegg. I like them because I take the approach on putting in a lot of fans running slower (some sucking air in [bottom and side] and some blowing air out [top and back]) to keep the noise down. I adjust the speed back to a tolerable level and leave it there. Three of these from newegg are $30.00 delivered. I've also heard a lot of people swear by the Stealth fans, which are slower and quieter out of the box. They are not adjustable, but apparently you would need to anyway. Read about them
here. Newegg has them, but three of them will cost $35. CoolerGuys is having a buy 2 get one free sale, so even with 3 day FedEx shipping (like Newegg), it's only $33.
Check them out here. Another brand people swear by in the Panasonic Panaflo fans. Whatever you go with, get ball bearing fans. They do last longer. Oh, and buy a spare. I've replaced more fans on my computers than any other part. Probably more than all other parts combined.
-cap