If you have any sort of an interest in college football at all the NCAA Football series made by EA is an absolute must-have. I'm a huge Nebraska fan so this series is one my favorites. NCAA Football 2003 is my second favorite game on the GCN right behind SSBM. Madden is great too, but personally I prefer college over NFL in real life so obviously I like NCAA Football more than Madden.
BTW, I would REALLY like it if they made a quality NCAA basketball game for the GCN. There has been like a total of one college basketball game throughout Nintendo's console lifetime and that was some crappy Fox Sports Net game. I realize Sega made one last year, but they started taking them off the shelves after the release and proceeded to cop out of making sports titles for the GCN so they don't count.