Recently, I've decided to play through a bunch of the older (good) Sonic games. Thus far I have checked off:
Sonic 1 - Complete with all emeralds
Sonic 2 - Complete with all emeralds
Sonic 3D Blast - Complete with all emeralds
Sonic 3 & Knuckles - Complete with all master emeralds
Sonic Adventure - Complete with all emblems
Sonic Adventure 2: Battle - Complete, 168/180 emblems (missing some Chao emblems and Cannon's Core)
Pretty proud of this list.
Back in the day I played these games a lot, but I often shared save files with my brothers or we took turns, so I wasn't sure which ones I had completed entirely on my own. I wanted to play through them again so I could say I've done it with confidence (and just because they're good games). Hoping to finish up SA2 some day, although the Chao emblems take a while... I'll probably add on Sonic R to the list, maybe the missions in SA
X, then stop there; I was never a big fan of the games after the Sonic Adventures, besides Sonic Generations which I've already completed 100%.