The shift to a hip-hop new jack swing sound for 3 was a great shift for the series and totally fitted it. It's also no surprise that the MJ stuff is the best in the game, though Hydrocity Zone (particularly Act 2) is beyond brilliant. And Kuckles has flying Battery which is godly. I also liked the addition of having different tracks for each act as it provided some extra variety.
I wouldn't say 3 is a clear winner. Given 2 has stuff like Metropolis, Chemical Plant, Aquatic Ruin, Emerald Hill, Mystic Cave, etc. But I rate them both very highly as game soundtracks.
Sonic 1 is all class too, but it doesn't quite match the compositions of its younger brothers. It does have Starlight Zone though.
As for 4, from what I've heard I'd say the compositions themselves are pretty good, but god some of those synths are too retro for their own good. They actually manage to sound more dated than the Genesis games. Melodically though they're pretty solid.