Scythe GTs are great if you're not too anal about the color choice, they work really well for radiators.
I am quite anal, plus they aren't sold in America anymore
I'd go for the Corsair SP120 Quiet Edition or High Performance Edition (they've been recommended by XBitLabs). They come with an interchangeable color ring, which can be red, blue or white; the high performance edition is just running at a higher fan speed, so the one you get should depend on whether you care more about temperature or quietness.
Linus Tech swears by the Noctua NH-P14 FLX 140mm. I suppose they make a comparable 120mm fan.
Running these in my case and on my Water 2.0 Performer:
COUGAR CF-V12HPB
http://www.newegg.com/Product/Produc...82E16835553006
Very, very happy with lack of noise and performance. I'm able to run them all at only 800 RPM, so there is a lot of headroom if I need to turn them up.
I second the Cougar fan's, that's what I'm running on my Corsair H100 radiator.
Approx $40 a pop for a Noctua - Sets the Industrial STD.
What do they cost you?
Noctua NF-F12 PWM 120MM NCIX Cda: $25 + Shipping and Tax = $35+: http://www.newegg.ca/Product/Product...82E16835608017
Noctua NF-P14 FLX NewEgg Cda: $30 + $10 shipping & Tax = $40+: http://www.newegg.ca/Product/Product.aspx?Item=N82E16835608017
SO MUCH FOR FREE TRADE
What about Noiseblocker Multiframes (black frame and clear blades)? Also very, very good, and just less than $30 each with shipping. Otherwise, the Corsair SP120 (either kind) are probably the best you'll get for "cheap" ($30 for a pair). They should improve performance over stock.