Sonic Adventure 2: Battle has been a staple through my childhood, and continues to be a game I keep playing on the GameCube. There's so much to do in the game and it has such a great storyline that it's really hard to get bored with this. It's challenging, fun, and has you curious to know what will happen next.
I love how the game gives you the option to play as either the heroes or villains. They're both pretty much the same story, but you get to watch it in different perspectives. And by being able to play with 6 different characters, it definitely keeps things interesting.
I've always been a fan of the Sonic games, but one of the main reasons I liked SA2B is because it involves more than just running/racing. Sometimes the Sonic franchise can get boring for me if that's all it entails. Not only can you do this, but you can also treasure hunt with Knuckles/Rouge and fight in machines with Tails/Dr. Eggman.
If you beat the story, you also have many other things you can do, such as complete other missions on levels, go kart racing, or raise Chao in the Chao Garden.
The Story Mode is not super long to be honest, however if you're goal is to unlock everything possible it will surely give you a good amount of gameplay.
This is definitely my favorite Sonic game that I've ever owned...I never want to get rid of it!!