Personally torn between deeps and heals. By deeps I mean proper deeps (50k+) and by heals I mean proper group support (Shards/Orbs/Drain with decent PotL/Warhorn/Prayer/SPC uptime and WoE for Exploiter etc.).
Difficulty-wise I'd arguably place it like this: 50k+ DPSer > healer > tank > just DPSer
(Healer is also #1 to blame for tank not taunting and 1-shoted DD standing in stupid.)
wow im surprised to see all the tanks on this poll - where are you all when queueing for random dungeons?