Which one is the best Sonic game and why?

Generations. Far more 3D sections compared to colors, and a lot of the 3D in colors was kind of weird on railsish sections. The 2D sections also worked better without the wisps.

Neither.

They don't even come close to touching the 2D Games, and I wouldn't even rank either of them as the best 3D game. Give me Adventure 2, Heroes, and Unleashed anyday.

But between the two, I'd go with Generations.

Generations. It’s not even a debate. They take the best of 2d and 3d levels and nostalgia into one package

Colours is better.
The 2D sections were more consistently good, and level design in general was more polished. Those special levels in Generations were a bit of a drag also.

Generations cause I never played colors

Sonic Colors for me.

Generations while cool was just a Nostalgia game but the levels were not that great in design. The 2D sections remixed the old levels making them worse. Without nostalgia there is not much to this game. Sonic Colors is the only 3D Sonic game ever made that is not only original for the series but genuinely great. A great OST. Well thought out levels with fun platforming. Pushed the Wii to the point of a game that would look stunning in HD at the time. How can we forget this?

I love the aesthetic and atmosphere of both versions of colors. Generations is nice, but it's more fanservice than its own thing.

Generations for me, really enjoyed it.