tough call, each site is different, over all I would say Google, then Yahoo, then MSN, BUT lately with some new sites MSN has provided much better results, I guess for now though I will go vote google
MSN because that was the easiest one to rank on, I was ranked #1 for a long time, so that definately brought me the most, followed by Yahoo, and a little bit by Google (but I expect it to go way up soon).
MSN for a little while but Google stepped up and took over. The funny thing is even while we had such good rankings in MSN we still got more traffic from not nearly as good rankings in Google.