It's got to be F-Zero GX for me.
Back when I worked for WalMart, I put a copy of this game in our GC display case. I don't think we'd sold a single copy in the first three months of its release. On my lunch break, I'd hang out by the game corner and show off my skills on the game. Soon, I was attracting large crowds, and a lot of people were buying the game(Many with Gamecubes). After it became a Greatest Hits title and dropped to $20, it was one of our biggest sellers. I did the same thing with NFL 2k4 on PS2, and that outsold even Madden at my store :lol