I am not a huge Sonic fan myself. But I do own all the Genesis ones, SonicCD, Sonic and Knuckles, and Sonic Adventure 1. I have never played Chaotix so I can't comment on it. But of all the Genesis Sonic games I still fell that Sonic 1 is the best Sonic. All the others are just WAY to cluttered up. The level design makes little sense half the time. SonicCD is by far the most jumbled Sonic I have ever played. They could have made the game a lot better if they did not try to cram as many platforms and loops into the level as humanly possiable, and just make it clean and naturaly difficult. Sonic 1 IMO has a nicer level design and is more enjoyable then the others. I don't hate the other Sonic games, they are just not as fun as the First.
