First off I just wanna say that I played DX with the mod pack that fixes a lot of the issues the ports made while keeping the stuff it added and did well.
That being said, this is probably my favorite Sonic game lol. The environments, levels, and atmosphere are just unmatched, and unlike most any other game in the series. The controls still hold up extremely well, and yes I'm even including the Big stages (literally look up a guide on how to play them, it's the easiest thing in the world when you figure it out and I'm not even joking.)
Ive seen some people say this game is only good for the Sonic sections, but that feels super disingenuous, as I felt every character had at least one great level, and watching the story weave together with all of these seemingly loose threads and plot points, coming together to tell a satisfying narrative about learning from the mistakes of the past, was fantastic.
The world also just feels fully realized, there's so many NPCs in station square and the general overworld with constantly changing lives and dialogue, you're rewarded for paying attention and keeping notes on all of these citizens. This feeling is compounded by how fucking good the atmosphere in this game is, its just so easy to get sucked in and play for hours on end. What a good ass game. I could go on and on about how I love pretty much everything about this game, but i ain't got that kind of free time on me. You owe it to yourself to play the original dreamcast version of sonic adventure, it really is an adventure like no other.