Sonic Before the Sequel
I saw the 2012 version up during SAGE 2012, and I figured why not. It's a very good fangame! The meaning behind 'Sonic Before the Sequel' means that the game is supposed to take place between Sonic 1 and Sonic 2. Fangames usually differ in terms of they feel, especially in terms of what Sonic fans like to scream out every now and then: physics. While not perfect (some floatiness), SBtS's physics feel pretty good. Most of the level design revolves around trying to make Sonic move faster and it doesn't seem to like throwing obstructions in front of you to stop you from going quick in earlier stages. In later stages, of course, they get a little tricker. You also move between playing Sonic and Tails despite them having not met yet. Tails's levels have less running sections and more platforming, but that's because he flies. There's also a Yoshi's Island 'touch fuzzy get dizzy' type of thing in Tails's levels.
By the way, the 2012 version has some good music (ex:
Sunset Star Act 3;
Metro Madness Act 1,
Fortress Flow Boss,
Lost Levels Act 1). I sometimes feel like the art is a little cluttered in the levels, but otherwise, it plays well. It is longer than you think it is, though.
You can download it
here if you're interested.