I say it isn't.
Sonic 2's levels are based a lot more around speed then anything else. It has a lot of unfair traps that you have to know are coming to have a chance of avoiding (prime example, on Sadv2's cyber level, there is a speed booster that sends you running up a wall, then gives you next to no time to jump on to a platform before you smash into spikes. It's bosses have some really unfair OHKO moves, and it's special stages are nigh impossible to enter, let alone beat. What's more is it requires you to defeat the game, chaos emeralds and all to unlock features that are already available or easy to unlock on the other two games (such as the tiny chao garden, which requires you to beat the game with two different characters, when Sonic Advance gives you it from the start).
It might be because I suck at video games, but the game seems mostly unfair and based on fake difficulty. I'd vote SAd2 the worst out of the trilogy, maybe out of the 2d games in general.
I'm interested to know why others say otherwise though.