Sonic Adventure is pretty good
I played Sonic Adventure and Sonic Adventure 2 and I thought they were pretty good but I much preferred the first one over the second. Adventure 1 being produced by Sega and not Sega of America was partially why I feel this way, the vibes in sa1 are much more late 90s japan vibes rather than sa2 which was very Americanized with Shadow the Hedgehog (not necessarily a bad character but he is notably poorly written even for a sonic character). I really liked the hub world structure of sa1, while the level structure of sa2 left much to be desired with a simple level select. Personally one of the best changes from sa1 in sa2 is the updated chao garden where you can raise chao in up to 3 gardens with a fully featured kindergarten for your chao, including naming, learning, and black market services. The difference between the chao garden in sa1 and sa2 is stark, with sa1's base dreamcast version having 3 gardens that are separated by space in the hub world, and chao alignment (hero and devil chao) being unimplemented excepting the alignment effects of fruit. Aside from the changes in the chao garden sa2 some changes I particularly despise, mostly concerning the change in playstyle: in sa1 the game was played as sonic all the way through, then played through character's stories starting from the point where they appeared in sonic's story, in sa2 the story is played through in two story threads that are intertwined, good and evil stories. The major mechanic difference manifests itself in the way that the games are enjoyed, in sa1 the story is played through with each character having a gimmick with sonic being the most fun in general and big being the agreed upon worst, in sa2 the good story and evil story have characters that are essentially mechanical clones in both stories (sonic/shadow(speed), tails/eggman(mech), knuckles/rouge(treasure hunting)). This makes sa1 more fun to play in my opinion, because you can play through stories at your own pace, while sa2 requires you to play through a linear story TWICE with clone characters. All in all I think sa1 and sa2 are both great games and would encourage you to buy them cheap from a sonic steam bundle (like from humble).
(also cannons core is the most fucking bullshit level ever and I skipped it (I even beat the biolizard after I skipped it so don't say skill issue because I don't have a skill issue I just fucking hate mech stages))