Of the true classic games, I'm not sure. Each of them obviously have their own hangups, but I think I'd probably give it to Sonic 1. It's got that 'first game in the series' weirdness concentrated pretty heavily in the Special Stages that have only been done since as homages instead ofna true evolution of the concept.
If we count Mania as a sort of neo-Classic, then Mania takes it pretty easily. Well crafted, fun, many different ways to play it. It's a great idea that lends itself well to Sonic.
If we're count fangames that emulate Classic, then my true personal favorite is Triple Trouble 16-Bit. Racing Fang to the Emerald, having to hit boosts, dodge his attacks, hit him back, all of it incredibly frantic and keeping you on your toes. The Fang races are so great that when I got the last Emerald I was disappointed that I couldn't keep doing them.