Sonic Adventure 2 (2001)
Sonic Adventure 2 was (obviously) the sequel to Sonic Adventure which was a launch game for the Sega Dreamcast. Both games are 3D platform games featuring Sonic and the first Sonic Adventure was the first Sonic game in the main series since Sonic & Knuckles on the Genesis.
Development started almost immediately after the first Sonic Adventure and took approximately 18 months. Like Sonic Adventure, Sonic Adventure 2 was released first on the Dreamcast and then on the Gamecube months later. Sonic Adventure 2 was also re-released in HD as downloadable content in 2012 on the Xbox 360, PS3 and Windows.
I was always more of a fan of the 2D games myself but if you are a Sonic fan then these are worth playing as they were some of the best 3D platform games out there during that era.