Zul'Farrak vs. Zul'Gurub vs. Zul'Aman.
Bronko on the forums says "if the mobs were of equal level... go!" but I can't tell if he means which we like better or which would win in a throwdown (my hunch thinks "throwdown," since that would be so much more interesting to see). So let's answer both.
I haven't played Zul'Aman yet, so it may be amazing, but I think I might have to agree with Bornakk-- the stairs in Zul'Farrak are pretty epic, especially for their level. Then again, Zul'Aman will probably have at least one or two events like that, so I'd be partial to the troll city in the Ghostlands in that case. And ZG is definitely a great
10 man 20 man (boy it's been a while since I've been there), too-- Hakkar's whispers, all the different bosses, the funny little in-jokes ("Ding!" "Grats!" is a classic, as is Nat Pagle's fishing spot), and the different animal areas make it a great instance. Hopefully Zul'Aman will match up.
And in terms of a throwdown, as players note in the thread, the Blood Drinkers in Zul'Gurub make things harder for everyone else. Will the trolls from ZF and ZA know to down the other bosses before Hakkar? Or can anyone, for that matter, stand up to the might of Zul'jin himself? So who wins, ZA, ZF, or ZG?