In the classic era, Sonic CD is one of my favorites- being one of the few classic games I managed to beat (I suck at classic Sonic
), its music, stages, and animated cutscenes have taken my retro heart.
The Dreamcast era is Sonic Adventure 2 Battle, hands down. SA2B was the first Sonic game to hook me into SEGA after years of being a Nintendo fan. If my Wii was hooked up, I would be playing SA2B daily- that's how much I still love this game. It has its flaws, both gameplay and story wise, that I can admit. The game doesn't hold up that well when compared to Modern Sonic (and at the same time, kinda does?), but I still find enjoyment out of it.
Modern era has Sonic Generations. I admittedly haven't played most of the modern games, due to observing multiple gamebreaking glitches in them (Sonic 06, Sonic Boom, ect), but Sonic Generations was a wonderful breath of air when compared to the other modern games.