Actually, I should say that I can't quite pass judgment yet -- I haven't visited the Halls of Stone or Lightning yet, and I haven't yet seen The Oculus, which is the instance I was most excited about at launch. But so far, I have to say that Azjol-Nerub is a standout -- not only does it look great, but the fact that it's pretty much all bosses and no trash is a real plus. And having a well-known lore figure to fight at the end doesn't hurt, either.
And of course I remain a fan of Utgarde Keep, though I think it'll eventually be like Hellfire Ramparts, in that I'll just have done it so much eventually that it'll lose any charm I have now. What are your first impressions on playing through all the five mans so far?