Sonic Adventure 2: Battle on the GC was not only one of my most favorite games on the GC, but probably my favorite Sonic game ever, and I'm a pretty big fan of the sonic games, even the newer ones that most others don't like. A few reasons I liked it:
1. A hell of a lot to do. Had all the different characters you could play with and so many emblems to get. Felt like playing a 3D Mario game, and I think that was the intention of the game in the first place.
2. Soundtrack. Music in that game is some of the best I can remember. Sega did a great job with that part.
3. Just playing through the story. I loved all the different kinds of missions and figuring out what was going on. Wasn't the best story by any means, but I enjoyed playing both sides and seeing what happens in the end. Like the first one, it just flowed really well. Not to mention all the side stuff you could do just added to the fun.
4. Chao garden. I spent so much time there haha.
Sonic Adventure 2: Battle was just pure fun to me. Maybe not technically an amazing game, but fun factor was really high. And I sure as hell got my fun out of it, because I've played that game for well over 200 hours and that's probably cutting myself short.