+1 for the Schwalbe Marathon Plus.

Last fall, I bought a pair of 700x32 to replace some slightly knobby, worn out Kenda's. What a HUGE difference they made in the comfort of the ride, and I actually gained about 1-2 MPH, probably because they have a much slicker tread, and less rolling resistance. But they are heavy, 850g EACH. I've put 2100 miles on them so far, and they still look almost brand new, and haven't had a flat yet (knock on melamine covered particle board).