Labyrinth Zone, as a child I think that was the most terrifying level from any game that I would try to endure. Without the spindash Sonic feels so sluggish underwater, combined with the claustrophobic design of the level anyway, the fireball enemies where you either had to wait to attack or just avoid them, the rotating spikeballs, arrow traps, occasional switches and a single dastardly spike trap placed underwater at one point that crushes you if you fail to pass quick enough. Everything forced you to take your time underwater, when really that was the last thing you wanted to do.
.....And then the drown countdown kicks in and everything becomes even more scary.