As someone who started with the Genesis games back in the early 1990s, I'm going to be a bit blasphemous and say that SA1 is in my top 5 favorite Sonic games. I have an absolute blast playing it, and while there are clearly flaws in execution, I think it has the right balance of gameplay elements and focus on speed.
I also think presentation is important, and the look and sound of the game exceeded my expectations for 3D Sonic. Seriously, locations like Emerald Coast and Sky Deck are up there with the greatest and most iconic Sonic levels in no small part because of the look and music.
I also love the hub worlds.
One thing that people forget--and I like to remind them--is that in the wake of Mario 64, almost every 3D platformer emulated it. It's a fantastic game and I absolutely love it, but I was also glad Sega tried something completely different with Sonic. They deserve credit for making a different type of 3D platformer--one that only Sonic could pull off--and adapting to it with moves like the homing attack. At the time, I don't think there was anything quite like it on the market. There still isn't.
Sonic Adventure 2 is decent, although the less-fun levels take up 2/3rds of the game as opposed to 1/3rd like in SA. Not a fan of any gameplay outside Sonic/Shadow in SA2. I also don't quite understand why they got rid of the overworld areas, which provided a nice break in the pacing--or at least a much better one than having to play as two monotonous characters between fun Sonic/Shadow levels.
Guess I just never had that much fun with SA2, though I recognize it's a lot more polished than the first game. For all its lack of polish, however, I still love SA1.