Of all of the games, this one is my favorite. Unfortunately, it is a port.
The chao feature is amazing. I have been playing this game for years and I have yet to be bored with it. Chao are virtual pets, and although it may seem strange to think that one of the game's shining points is a virtual pet game, it is very fun and is a lot more complicated than you may think. The gameplay is amazing, like most other Sonic games' gameplay. The music is great. I can tell that they really put effort into it. I often find myself listening to one of its many great songs. The main bad part about the game is the fact that it is almost exactly like the Dreamcast version. There are not many differences between it, besides some chao things.
Overall, I think that this game is a classic game that reminds me that Sonic Team once put effort into their games. I recommend buying it if you have a Gamecube. It is a player's choice game, so it is cheap ($20 or so) and worth it.