porTendrils for D, Psyfiend for O imo
Then for arenas it depends on playstyle
edit: dominate mind if you want to mess around and mc people off cliffs
I disagree with the Tendrils vs. Psyfiend arguement because, even though you have melee rooted with Tendrils, they can still throw Avenger's Shield or start autocasting Throw and, they may waste a Shield Slam or Avenger's Shield Cooldown on Tendrils, but they could do the same trying to kill Psyfiend and end up getting feared too. Also, when you are feared you have no control over your character, unlike when you are rooted.
I think Psyfiend fails a lot compared to Tendrils. Not as reliable, and I like to be confident that the spell will work.
It usually works if you position it right, even if the enemy players are smart they will keep a 10 yard distance on you and the Psyfiend. Leaving you time to heal or Dps, etc.