Overall, this expansion ranks way up there as having some of the best 5 mans in any xpac. Probably the best ones aside from Wrath for environment, layout, etc.
I voted the Arcway, but clicked a bit too hastily. RtK is a long needed return to proper dungeon design (at least before it was sectioned up for group use). Just an excellent, long dungeon that served to be very entertaining.
Arcway gets a nod for its vibe, and boss variety. It had (to me) the most Legion 'feel' to it.
VotW was the worst this expansion had to offer.