It's really only the Xbox that I can think of which didn't attempt to have a platforming mascot. Even the Playstation touted Crash Bandicoot as one of its biggest system sellers, though it was FFVII that won the gen for them.

Sonic & All-Stars Racing Transformed.
Why haven't we had a current gen game in that series yet

Tbh I think that sonic could've have a chance to stay good if '06 wasn't garbage.

Also, I think '06 could be good if it was tweaked and fixed

Exactly.  Mascot platformers declined in relevance. 

So, the peak was Sonic 3 and knuckles.

The peaks were all the 3 first games, Sonic CD and the Sonic adventures 1/2 (even though they didn't age as well as the 2D games).

The rest is pretty weak, even generations, wich didnt manage to perfectly capture the essence of either 2D or 3D Sonic completely (though the 3DS version kinda did and is overlooked), but it's accepted because of how bad it is these days.
Sonic 06 had potencial but they went and ruined that with a dumb story and rushed buggy game. The sonic 4's were too small and contained to leave any lasting impression. But i did enjoy both episodes (more the second).

True

Sonic Advance 2. Always.

Sonic even with bad games stays somehow very popular, the archie comis is still going on, is smeag buying them all?

Sonic 3 and Knuckles were the last great 2D sonic games imo. You could argue Sonic Advance but even that doesn't compare. In terms of 3D, it peaked at Sonic Adventure 2.

Heroes was fun, 06' never played it either. I'm pretty biased towards sonic games though xD. I enjoyed all of them honestly. I can see the obvious flaws of them, but they don't ruin the games for me :P