Hydrocity has always been a top 3 Sonic zone for me. As much flak as S3 gets, I loved that game so much. I liked it far better than 1 and 2, even without the lockon. I think S3 takes longer than 2 as well, it has less zones but the levels are far bigger.
The music was crazy and nearly every Sonic 3 zone is excellent. Marble was probably the worst but its still better than the latter half of S2, and carnival had the barrel, but the rest is top notch. Angel Island, Icecap and Hydro are among the very best.
Sonic 3, the complete edition, is probably the best Sonic still. Its the most cohesive in level design. Mania eventually drops the ball when it comes to stage transitions. As for Knuckles campaign, in Mania there are a few levels tailored to him but its largely identical otherwise. In S3 the entire game was altered for his sake. He had much more different bosses (and basically no Eggman, as his campaign took place at a later time), his final boss was completely unique. But really, its like expecting Nintendo fans to create a game that beats Mario World.
I think this game beats Sonic 1 and 2 and thats quite an achievement for something thats pretty much a fan project.