Well if you want a solid platformer I can't really recommend anything but Sonic 3 and Sonic & Knuckles. If you buy them both you get the option of playing Sonic 3 & Knuckles which is in fact what Sonic 3 was originally going to be. But due to high cartridge costs for games that size they cut it in half, and have done ever since by a greedy and lazy SEGA . But at least they kept the connectivity between the 2 in place for the XBLA release so you can still play the full game. It just costs twice as much as Sonic 2 as a result... But it is much much bigger than Sonic 2 so its all good.
As for other games, I don't really know. I've never bought an XBLA game, and on friday I bought my first ever MS points (I hate the MS Points sytem with a passion) because there's upcoming things that I want. The Perfect Dark re-release, Sonic 4, and the Ballad of Gay Tony are my current 3 options to spend the points on. I don't know which to choose.