Ah, Rifts - I had some fun in that, ran three groups simultaneously with around 25 people in them, all against each other. When that campaign wrapped up, someone (who was sick of me talking about how Rifts was filled with munchkin characters) volunteered to run a game, and wanted us all to make the most munchkin characters we could scrounge up.
Then he said, "Oh, if you find something neat on the Internet (this was '98 by the way), run it by me, and I'll probably let you do it."
It got BAD, real quick.
I'd have to put a vote in for GURPS, actually - GURPS. The name of the game even SOUNDS like something you'd do while bent over a toilet. It's a rules-heavy mess.
One of my friends was running a superhero game, and I wanted to be a mage. "Okay," he said, "here's the book!" I larned me the rules, and just starting out my character had some 30 spells, most of which he had an 85% chance of succeeding on with only a single point in 'em. Kinda bad, having that many 'powers' when the other players only had five.
Though looking at some of the other games suggested here, GURPS isn't THAT bad.