Sonic the Hedgehog’s Hydro City is one of the best level designs in any Sonic game.
Hydro City features two acts, of which the second is probably the better. It’s also the only level in Sonic 3 with air bubbles.
In Act One, you land in a massive water pool and have to navigate around pits, enemies and spikes, like in most Sonic levels, but it’s harder because of the water mechanics.
Some areas of the level allow you to gather speed so you can race right through the zone if you have the skill. There are breakable walls that lead to jets of water that push you forwards.
In Act 2 a wall on the left hand side of the screen constantly pushes forward, meaning you have to race. This makes the level very hard, especially as you have to take oxygen into account.
Take a look at one of the best levels in Sonic 3 with this video.