I couldn't get past Sonic 1.
Why?
I couldn't get past Act 3 of the Marble Zone in Sonic 1, at first, I thought act 2 of that Zone was the hardest Act ever, since I couldn't get past it(even w/ 0 lives at the end of act 2, of course), until (I think) April 2005, 1 month before I got Pokemon Emerald, but that was 11 months ago... When I got past it, I had 0 lives at the end of act 2... Then I found out that Act 3 was even harder for me, when my character's last life went down the drain due to the difficulty, meaning... GAME OVER! But, I know that I could be wrong... In my opinion, the first three(if you don't count S&K) Sonic games that came out for the Genesis have at least one hard point in them, with one of those hard points in first one among those games rock-hard, but not as rock-hard(but close) as the final level of the Super Return of the Jedi game that came out for the SNES(if you heard about it from Gamefaqs, not even the most skilled video game players can beat the final level of that game), but that's another story...