SA2 is considered by most as one of the coolest Sonic games of all time. It removed the things from SA1 that people didn't like so much, such as the adventure fields and fishing as Big. It also introduced two new characters, one of which sparked a fanbase for him alone...Shadow. Shadow, a mysterious Sonic look alike, may be the reason that SA2 did so well in stores. Now when you ask someone to name to Sonic characters, the response is usally 'Sonic and Shadow' unlike the former 'Sonic and Tails'. If you don't believe me, test it for yourself.
SA2 also has what fans call The 180 emblem challange. It is near impossible to pull off, but there are a total of 180 emblems in the game. If you can get them all you get the best reward of all time: 3D Green hill Zone (see the movie section for a peek at it).
Combining a great soundtrack with an excellent story, plus Shadow, equals a great game. Plus with all the bonus features, it is a huge game that has lots of replay value.