Hmm, probably "And My Name's Booster" from SMRPG. There's a great remix of it floating around on the Net called "Booster Tarantino," too, but I think I actually like the original better.
Other than that, pretty much everything from Yoshi's Island, especially the title theme (after reaching world 6) and the ending credits theme. You have no idea how much I'd pay for that OST. (Well, if I had money.)