If overgeared, all dungeons are very doable. But, imo, in the normal process of leveling, with gear we got from dungeon drops with no enchants or boosts, the most difficult or least boxer friendly are:

The Oculus because of the mechanics - need some particular macros/actions.
Stonecore because of huge trash packs with strong aoe. Bad pull or some mellee running out and that's a mess.
Throne of the Tides - 1st boss is a PITA. Just made it when my teams reached lev 85. I've never beat this boss with a 80-84 team.
Stormstout Brewery - I found last boss to be one of the most challenging fights I've done while multiboxing. Actually I'm still learning the best way to deal with it.