What games should I play?

I've been a long time sonic fan since I was a kid basically. I played Sonic on my old DS (that's sadly gone now) and barely Sonic Unleashed on my cousin's Xbox 360 when I was younger. Now I've recently bought Sonic x Shadows Generations and it's gotten me into my Sonic phase again. Wondering what games should I try to buy (when I can) to play on my Xbox next. I've only played two/three games now and haven't completed any of them and I feel like I need to play a bit more if I want to call myself a fan of the series. Any suggestions or help?