So which is the best?

Well I have fun on my Fire/Elec/Mace. I don't like the fact that I survive on greens and don't have a phase power; however, he hits like a ton of bricks when it comes to unsuspecting squishies in RV and I can easily set up a stalker's Assassin Strike with Ring of Fire.

Mace - You're in this pool for added damage as well as set bonuses. It is a good place for another two Luck of the Gamblers and/or Karma +Knock Back protections, and with the pet you can get the other three Soulbound Allegiance IOs slotted (Or just three in the case of Mind).

Leviathan - This set has a good shield and Hibernate. This set also has a place for Soulbound Allegiance IOs and Water Spout (Opinions vary about the Spout).

Mu - It has been a while since I have used this patron in PvP, however I find it more suited for arena fights. It has good endurance draining capabilities to supplement Drain Psyche, and has a good shield. The kicker of this set is the pet which can be slotted with Soulbound Allegiance IOs AND is a healer (In today's PvP world I don't know how good he is at healing I'm afraid). The God Mode is a little tricky to use, however if timed right Domination can be used to basically undo its crash; however, Diminishing returns needs to be brought into the picture when considering this power's usefulness. Note: Power Surge does have +recovery.

Soul - No. (I forget if Tactics + Rectified Reticle gets a Dominator to the Perception cap. If it doesn't, it's interesting to note that the pet in this set has a +perception AoE toggle that she keeps on iirc.)