The first 3D Sonic game is probably the best one, Even argubly the best Sonic game full stop?
Sonic Adventure was an immediate sucess due to the Blue Blur's popularity and luckily enough it was a amazing game to match
The graphics at its time were unrivaled even by todays standards some of the CG sections look immense, In game they also looked terrific. In the Lost World level Sonic Team used photo's from a temple they visited to use for the level.
The music in Sonic Adventure is just as amazing as the graphics boasting full rock based soundtracks with level music that had some feel to it that really fit into each level.
Sonic Adventure plays as you would expect, You take the role of the Hedgie himself and 5 other characters to stop the evil Dr. Eggman from destroying the city and building another city 'Robotnik Land' so you go on a mad quest to collect the Chaos Emeralds to save everyone, jumping, rolling and the rest of it, Except for a few flaws in the game engine where you would fall through the floor randomly at times, Its fine.
Most people also seem to find the camera annoying but after a few playthrough's of a level its easy to learn when it changes and move the analog stick accordingly.
If any 5 games were worth owning for the ill-fated Dreamcast then this is one of them the 2 major Sonic games on this system were in some peoples opinion the last good Sonic games. Overall if you have the console buy Sonic Adventure is all I can say there is a few flaws but otherwise its too good a game to miss out on.