Gundrak is my rng's dungeon of choice. I have copped it, finished it then switched to another toon and got it again TWICE.
But apart form that one dungeon I have a pretty even spread, Ive had OK, AN, Ocu, HoS, HoL, Nex, UK, UP, FoS, PoS, ToC and any I have missed except for HoR. None seem to appear a whole lot more then others, maybe HoS but not by much.