Caviar Black is my choice.... But I'd get the 1TB since your opening the iMac up and all.... for $30 the platters are denser and thus quicker.
Second would be the 7200.12 but only the 500GB since its on one platter its nice and quick, neck and neck with the black.. the 1TB is two and loses alot of the benefits.
I'd stay away from the blue.. thats the "value" line, the green, while a bit slower, is actually much quieter and cooler (due to lower power use)
I have a WD 1TB black in mine and no problems, no fans on more... nothing but better speed is all I can see :-)
p.s. Snow Leopard screams on the black!