Sonic 1 Alt (Sonic 1 Hack) by Eli678 - Full Longplay with All Chaos Emeralds
Sonic 1 Alt is probably one of the first Sonic 1 hacks to have original level design, though I could probably be wrong on that since it came out around 2010 and I'm pretty sure some Sonic hacks in the 2000s had original level design as well. I'll admit, I do like the ideas of the original level design here, since some of them require some strategy and slow movement. However, since I'm used to the fast paced level design of the OG Sonic 1, I did fail a bunch of times, sometimes I used rewind as well since there were some crap moments which gave me a hard time. The difficulty spikes though feel all over the place, with Scrap Brain being especially pretty difficult. This hack could be done without rewind or save states though, but it would require a lot of practice with the new level design tweaks. A shame almost all of the bosses haven't been altered besides the Green Hill one though, because it would be neat to have more difficult and strategic ways to defeat the bosses as well. Not sure if I would recommend this one, but for being a full level design hack (not counting the bosses), I'd say maybe check it out.
Download: https://info.sonicretro.org/Sonic_1_Alt