Sonic the Hedgehog 4. It's everything Sega promised it would be: just Sonic without all the cruft. I got to play through the game's Splash Hill Zone, and it's pure, unadulterated Sonic the Hedgehog 2, with a new coat of paint and a handful of new features. And there's no Tails, of course (not in Episode One at least).
There are a few new wrinkles. Sonic has his homing attack from Sonic Adventure, for example. As with most of the 2D Sonic titles, there are also a few interesting locomotion mechanics sprinkled throughout the levels: vines to swing on, ziplines and, of course, the beloved corkscrews and loops.