The M18 by a country mile. This was a brilliant vehicle. In mobility, it flew; damn near literally. It could actually out maneuver enemy vehicles ability to track and fire on it. It is also still the fastest tracked AFV ever built.
As a Self-propelled gun it was one nasty opponet. The open top made for a vehicle with the ability to go virtually completely turret down with a little effort leaving little to shoot at. Its smooth turret operation and fast rotation allowed a very quick engagement rate.
The designers obviously understood the tank-on-tank and antitank game very well.
Yes, the armor was paper thin. But, you can't hit what you can't see and what you can't track. And with the M18 both were more than possible.