I love Sonic 1, however I feel I was spoiled by the spindash and xxxtreme speeds of Sonic 2 and the epic plot of Sonic 3 & Knuckles. Going back to it after those, it's still an excellent game. But it feels slower and Sonic feels underpowered without all his later moves.
Still, I probably play through Sonic 1 more than any other Sonic game just because the zone progression is so natural.