Which RPG have you played the most in your life?
Dungeons & Dragons.
This is the game I started with. Sometimes I will either grow tired of it or want to try something new, but I always end up wanting to run a game with it eventually. Curiously I always end up playing the latest edition - maybe my desire for a new games is what keeps me moving editions.
Why D&D? I'm not sure.
Maybe there is a certain comfort level there.
Maybe the rules are similar enough that I can pick up the newest edition without a large amount of rule induction.
Maybe fantasy offers the greatest area to explore and keep things interesting.
Maybe because the players I game with also know the system.
Maybe because I like the fantasy genre the most and is what I read the most.
Not 100% sure why but I always end up going back.