I like playing all 3 but as a tank I can't really speed things up that's the DPS job + the healer if they have time to DPS my favorite DPS for dungeons is probably summoner I love destroying everything and using Bane + Deathflare. My heals go to is either Sch or Ast since I can bring the DPS on either or I can support a DPS on my Ast, Whm is in the works but it has Holy + it's own burst DPS so i'm looking forward to that.

My advice is just play what you feel comfortable/enjoy playing only you know what you wanna play, if you're like me who enjoy being a good DPS and pushing numbers/supporting the group then I understand completely we tanks can only do so much.