[*]Sonic Adventure DX (2003, GameCube): Minor graphic overhaul; everyone's now made of shiny plastic for some reason, and some textures got changed (YMMV on whether for better or worse). Water has fancy shader effects. Game tries to run at 60FPS, often failing to (with noticeable drops). Extra 60 missions added to make a total of 190 things to do (130 Emblems + 60 Missions). Metal Sonic added as a reward for getting all Emblems and completing all Missions (as opposed to the jack you get in the DC original). Game Gear games added as intermittent rewards for Emblem collection/Mission completion. Chao Gardens get serious overhaul, to basically be on par with Sonic Adventure 2: Battle's. Sonic X ads. Cream cameo. Font changes in dialog boxes. Sonic's eyelids are now blue (instead of the peach color they were in the original release), which will hold for all future games, all the way to the current day as-of-writing.
Basically, the game keeps getting subtly worse with each revision, with only the GC one being a debatable improvement.