In my early days of roleplaying, I'd play just about anything. I did spend more time with two games than any other besides AD&D. Those games were MERP and Warhammer Fantasy Roleplay. So, if I were to ask the question based on what I played back in the day, those two would be my choices.
If you were to ask me today, based on what I find myself wanting to play, it would be Tunnels & Trolls - a game I refused to play back in the day.
Go figure ;)
So, which game or games would make your list?