Sonic Adventure 1 is the greatest Sonic game I've ever played. The amazing 3-d enviroment was the best graphics I had ever seen at the time. Everything in the game looks amazing.
There's six different characters you can choose from, including Sonic, Knuckles, Tails, and a pretty cool robot. Each charater has their own levels, but the story correlates with all the other characters story. This adds to a lot of reply value, because when you beat all the levels with each character, it actually opens up the true ending of the game, and you get to play as ...I won't ruin it, you'll have to see for yoursel.
The controls and gameplay are good. The characters movements are very fluid, and each character has specific things they can do. Sonic can roll into a ball for amazing speed. Knucles can climb almost any wall and soar in the air for a while, etc.
The story is also done well. You'll have to stop Dr. Eggman from fiding all the chaos emeralds which have been scattered. He's using the emeralds in a mutant, water monster thingy, and each time he places another emerald in the monster, it gets bigger and more powerful. You'll fight Dr. Eggman and this monter numerous times throughout the game. Much like you had to fight Dr. Robotonik at the end of every level in the original sonic.
There's in adventure world where you can just mess around and do what you want, it's also where you find the new levels. And there's an action world, where the actual levels take place. Like any other Sonic game, this one is FAST.
All in all, this is one of my favorite games of all time. The graphics are still good for today, the story is good, there's ton of speed, and of course, it's SONIC!