We don't need any more Mario characters. We already have Mario, Peach, Bowser, Yoshi, and probably Luigi. That's five, 7 including DK and Wario. And even if there are more, they shouldn't be sucky characters like Petey Piranha.
Mario Kart 64: The courses were all amazing. There were so many shortcuts caused by mere glitches that actually took skill to pull off. Plus, the power slide boost wasn't cheap at all.
Mario Kart DS: Plenty of great tracks, though there were a few that flat-out sucked. However, the biggest complaint I have is that snaking is really cheap. The WiFi was done well, but they, unfortunately, only included 20 tracks and didn't have a cut-off button for friend searches.
Mario Kart Super Circuit: The track design was okay, and the physics were perfect. My complaint? The fact that you have to have two GBAs and two copies of the game to successfully enjoy multiplayer.
Mario Kart Double Dash: Eh, it was okay. Some tracks were fun to play on, while others were boring. The AI cheats way too much.
Super Mario Kart: This was fun when I first played it, but when you go back to it... It's just dull.
I'm gonna try and go through the Music Hacking thread from the old board and recover as many txt files as possible. Bear in mind I have not tested any of these in the hack, nor do I know who made them unless stated otherwise.