Yoshi's Island. My god how I miss that game. Easily my favourite. It's just soo cute. Not to mention fun to play too.

I've not played either of the Galaxy series of games, but they look awesome, and I'd love to play them. Mario Kart is just amazing, as it's just a communal, fun game. Of the other games, 64 is very good. 3D Land is fairly good, but I found it rather easy to be honest, easier than other Mario games.