1. Sonic 3 & Knuckles
The final form of Mega Drive Sonic created by literally jamming two carts together in what probably sounded like some kid on the playground's silly lie back in the day now I think about it.
Stages so dense I'm still finding new stuff each replay.
2. Sonic Colours
Boost style Sonic slows down a spell and embraces the Mario world of gimmicky powerups to surprising success. Also has the Aquarium Park music which is enough to elevate this so high on its own.
Really it's just nice to have a Sonic game that manages to hit both the speed factor and moments of slightly finicky but functional platforming.
3. Sonic Generations
Surprisingly a game of two halves where both halves are just as good as each other which is a nice change from split gameplay in other Sonic titles, see that's the bonus of having Sonic and also Sonic for your divide. Whisper it, the modern Sonic stages might just edge out classic this time around, a sign of the end of times to be sure.
Speaking of the end and time, Time Eater does its best to crap over the finale but you can't win them all.
4. Sonic 2
The realisation of Sonic 1's concept delivering fun times all the way up until either Wing Fortress or Metropolis depending on your own opinion, well I suppose you could enjoy both of those, I'm content to just throw in the towel after Metropolis myself.
All glory to the spindash.
5. Sonic & Sega All Stars Racing Transformed
Apparently Spinoffs count and well Sonic stuff still vastly outnumbers a bunch of the other Sega franchises combined so yeah, it's Sonic.
Sega Arcade style racing meets Diddy Kong Racing turned up to eleven, alas it does not meet Valkyria Chronicles representation at any point so it loses some points.
Adder's Lair is a standout race track.
6. Sonic Master System
Imagine a Sonic 1 where you retain that more focused platforming emphasis without the drudgery of marble zone and also get yuzo koshiro musical goodness, yeah the green hill ain't as strong as the Mega Drives but hey I'll gladly take the rest over it.
See also: non shitty bonus stages you want to visit for continues.
7. Sonic CD
Sonic Clusterfuck Disaster? Sonic Chaotic "Dinging"? Sonic Collectathon Dilemma?
Well anyway it's a bit of a mess but it's a fun mess to just blaze through without a care in the world, judging by some of the level design the same statement probably applied to the designers. Rad soundtrack on both sides but JPN/EU wins out even if US Wacky Workbench brings the furious funk.
...I don't feel strongly about any of the rest to list them, I feel like I could just say "well a third of Unleashed/Adventure/Adventure 2/Heroes/Lost World is okay" if that means anything.
I'd need to play the handheld ones again to actually remember anything notable about them, they tend to be like the greasy popcorn food of Sonic titles.