The first RPG I played for any length of time was ShadowRun, for a couple of years. Then I wandered off to uni, where I played a World of Darkness campaign for the best part of 3 years - I played other games too, but none for as long. (Actually, that was pretty great: I got to try loads of games, while still getting the long-running, in-depth games I particularly enjoy.) One was Aberrant (though I didn't know it at the time), mentioned now as we'll come back to that later, because this is RPG played most, not campaign.
Then I came home and played a mix of games, usually only for a few months at a time (this included some ShadowRun, but not enough to give it the topspot). We played a bit of Deadlands in preparation for The Flood, the campaign we're currently playing in, but before starting the campaign proper, we moved on to Pathfinder. We're now playing The Flood, but I think it's been around a year.
Then Rich also started running Aberrant for me and Bells, and we played for a good couple of years, but even adding the couple of months I played at uni doesn't do enough, because we then switched to Exalted, which we've been playing for a couple of years - so that doesn't get to win either.
It's going to be Pathfinder. It's far from my favourite system, but I love my main character.