Sonic Advance 2 (InstanceTopic, 19)
From Compile Worlds
Sonic Advance 2 is the sequel of Sonic Advance, running off a modified version of the latter's engine and starring Cream the Rabbit, so that must make it the best, right? Sure, if you like hit and run style boss battles akin to the Mushroom Hill Zone, crappy enemy placement, and Mode 7 Special Stages that rehash Sonic Adventure characters for marketing ploys. But beyond that, it's actually an awesome game, and features the most epic Super Sonic showdown ever in the form of the True Area 53.(Which isn't related to Area 51, mind you.) Since Sonic Advance is the only official candidate for a proper Sonic 4, this would technically make Advance 2 the only proper Sonic 5.
Also worth noting is Vanilla the Rabbit's first official appearance is in this game, and is the last game in the Gameboy Advance series to feature Dr. Eggman as the true antagonist.