Blackheart Haven and Ruins of Mazzutan.
Not because they are difficult dungeons on normal... I mean, they are a bit long for a random etc... but that's not the reason anyone wanting to level their war horns/barriers/blood altars or any skill they specifically put on their bars so when the random dungeon concludes all that juicy xp boosts their skills...
But because... you know that RNG will shine it's ugly torch right upon you and wipe your bar clean with the "Skeleton" mech in Blackheart or the "Statue vision" mechanic in Ruins... and the dungeon will finish with you getting ZERO EXP on your bars. It's dumb... it is... and it's the cause of many people who see these as their randoms, dumping the group upon entering the instance.