Hardest yes, but best? Insanely stupid level design and bosses, goofy sonic animation and pretty awful control.
Heh, I guess not playing a game in nearly 10 years will distort your memories like that. Sonic 1 GG is playable, but if it wasn't Sonic would we give it so much as a second thought today?
Although the game reminds me of one of the few times X-Play pissed me off when they showed that you could complete Act one of Green Hill on the GG Sonic 1 in about 30 seconds by just running and jumping once in a while. No kidding, that's virtually every first act in the first levels of every Sonic game.
Even with the new Sonic games, that's still the case. See SA1's Emerald Coast, SA2's City Escape and Heroes' Seaside Hill.
Especially the ending version of Sonic Boom.
And my favorite GG Sonic? Hmmm....I liked Sonic 1, Triple Trouble and Sonic Blast.