Sonic 3 & Knuckles > Sonic Mania > Sonic 2 > Sonic CD > Sonic 1 for me, if we only count the strictly Genesis/Mega Drive style games and not more "modern" entries such as Advance despite falling under generally the same gameplay umbrella.
1. Sonic Robo Blast 2. Its A Genuine Masterpeice, Extended By The Modding Community.
2. Sonic Mania (Plus) A Great Revisiting To The Classics.
3. Sonic 3 (Complete). The Sheer Amount Of Editable Stuff Allows For The Best Playthrough That Suits Your Likes.
4. Sonic 3 (A.I.R). The Widescreen And Unlockables Are Great (Also Somehow I Clipped Through The Wall In MGZ Act 1 And Beat The Act 2 Boss (Sadly Couldn't Hit The Capsule So I Had To Retry).
5: Sonic 2 (2013). Controls Are The Only Bad Thing About This.
6: Sonic 3. Was Really Lucky To Have This Before It Was Unlisted.
7: Sonic 1 (2013). This Is Only Higher Than The Original Because Of Widescreen.
8: Sonic 1. The Tighter Controls Lead Me To Beat The Labyrinth Boss And The Rest Of The Game.
9: Sonic CD (2011). I Feel Me Being Bad At The Game Contributed To The Blandness Of It (Also I Never Beat It).
10: Sonic Generations (3DS). The Small Screen And The The Slippery Controls Are Not The Best, And It Also Deleted My Progress About Halfway Through, Maybe I Would Like The Game Slightly More If My 3DS Screen Was Fixed.
if fangames/edits to original games count then it would be
1. SRB2 (no duh)
2. Sonic 3 A.I.R
3. Sonic 2 Absolute
but if they wouldn't count then it would be
1. Sonic Mania
2. Sonic The Hedgehog 2
3. Sonic The Hedgehog 3 & Knuckles
(the only reason I love Sonic 2 more was because I played it before Sonic 3 thus getting more nostalgia from it then Sonic 3.)