The games that ruined their series.

I too enjoyed SA2B, except for the Tails/Eggman stages. Definitely a step back from Gamma's stages in SA1. Enjoyed raising chaos. I didn't care at first for the direction they went with the stages in general (SA1 tried to stick true to Sonic levels), but I grew to like them.

Sonic Heroes - meh. It wasn't terrible, but not great either.
Sonic 06 - aaaha
Sonic Colours - I enjoyed it.
Sonic Generations - Really enjoyed it.
Sonic 4 - Okay. Uh. Why has it been nearly impossible for Sonic Team to get the rolling physics right? How is it that fangamers can make proper rolling physics, but a team of professionals can't? It really does bother me. Sonic is not supposed to be able to walk up 45 slopes like this. Rolling down slopes is not supposed to slow you down. My god. It was like that in Generations, but more bearable. I'm not a programmer, but I'd like to believe that if they could accomplish this goal in the 90's...they should be able to do it 15+ years later.

Anyway, /end rant. Out of these, I would definitely say Sonic 06 ruined the series that was already going downhill.

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Agreed on Dawn of Mana. It's a shame Secret of Mana 2 never saw a release outside of Japan.