Rosetta@home. But its TeamRAC is currently sky-rocketing [link], so expect it to become less mag-rewarding. However, Rosetta@home generates less heat than Universe@home [edit: theSkynet POGS] for example. I guess it's related to which CPU instruction sets are used. Didn't inspect further though.
Which projects are you using?
Iam with skynet pogs right now but iam not completely satisfied maybe iam going to change this quite soon.. iam thinking into the direction of WCG they simply have super attractive projects for me.
Ah, actually theSkynet POGS it was that produced more heat then Rosetta@home & Universe@home; you should try them out! Concerning WCG: don't expect to max out your mag with these - you will "min" it out.