SA2:B stages. They don't even HAVE S-ranks. But seriously...
In SADX, there are no "ranks", but the one emblem I don't have is sky chase act 2.
In SA2:B, I'd say one of the hardest to A-Rank is Crazy Gadget Mission 4 (it may have been my last A-rank, not counting green hill which I don't have because stupid chao)
In Sonic Heroes, don't ask me, I'm not even close.
In Shadow, they're all ridiculously easy.
In Unleashed (Wii-PS2), I haven't finished the game yet.