I have to say that the only thing Generations has over them (besides graphics) is the absense of gameplay styles which are a drag to play.
Sonic controls beautifully in the Adventure games, in Generations there's a certain button delay which has cost me my S Rank many times.
I've never found Adventures so much fun to play, I still prefer Generations and Sonic 3&K over them - they have better level design and less technicall issues.
There's no denial Generations and 3&K are superior games, I'm just saying that if the Adventure games were just Sonic and the SA1 Tails, it would at least surpass Generations.
Since the levels had to be suited for all characters in SA1, I prefer the slow and steady but gradually quick gameplay from it over the more reflex oriented one from SA2 and onwards.