To Adam Clayman's question of how the answers might change if I'd asked most interesting instead of hardest, my answer would be not much. Of the ones I've played that folks have discussed, I would rank these green complexes as among the best.
Other views?
Hmmmmm.... Adam raises a great question, as he so often does.
And I think in general I'd agree with John. How can greens and surrounds be the most interesting if they are also not among the most difficult? Isn't what makes them interesting their difficulty, at least for the most part?
Of course there would be an exception to this, though: greens that are so absurdly fast and contoured as to ratchet up the difficulty to the max, to a level of absurdity. Those would be the most difficult, but would be interesting only to masochists.
I can think of some examples... but in each case, all one has to do is ratchet down the speed a little and they work both as most difficult and most interesting.
Note also: I can think of greens that at least at first glance do not appear to be the most difficult, but are certainly among the most interesting. But wouldn't these be foolers, where the difficulty becomes apparent only after experience? I'm thinking of slowish but highly contoured greens.... Hard to say those aren't also among the most difficult, even at slowish speed. And they certainly are interesting.
TH