If we want to get super technical here
Sa1 = Sa1
Sa2 = Sa2
Sonic Heroes = sa3
Shadow the hedgehog = sa4
Sonic 06 = sa5
Sonic Unleashed = sa6
Sonic Colors = sa7
Sonic Generations = sa8
However only truly one of these games is a faithful Sonic Adventure game. And thats SA1. Too many changes and fuckups have turned the series into an unrecognizeable shit pile.
Sa2 = Tails in a mech, Stories are merged together
Heroes = Slippery shitty physics, Plastic Uncanny Valley models, forced 3 character gameplay, Shitty healthbar bulletsponge enemies
Shadow the hedgehog = same shitty bulletsponge enemies, same slippery physics, same uncanny valley models, conpletely removed every playable character including sonic
Sonic 06 = inproved physics over sonic heroes and ShTH, yet slower homing attacks, Amigo system instead of even whole levels for most of the characters, mach sections which ruin level continuity, elise sections which also ruin continuity of levels, levels broken into parts
Sonic Unleashed = Impressive boost system, with the drawback of shitty slippery physics once again, Fantastic hubworlds, yet removed every playable character again, except for a variation of sonic, made parts of levels 2D which was annoying
Sonic Colors = Made EVEN MORE LEVELS 2D, the 2player mode had Sonic robots ibstead of characters from the series which was a dumb fucking idea, shitty voice actor
Sonic Generations = Not only is there more 2D bullshit, they added 2D sections to normally 3D stages as a fuck you to modern fans, they once again removed all playable characters and added a variation of sonic, a fat pudgy one...
The hubworld was a 2D piece of shit and very low effort.
Basically everything that made adventure originally good they stripped away until it wasn't adventure anymore