Spiritual if your build is on the low end for recharge. It wont boost domination but it boosts hasten which will help with getting domination back.
Since I have enough recharge to be just shy of perma hasten by a few seconds I went with radial intuition for my Mind/Fire. +Hold/+Damage/+Range all worked well for the toon. It's softcapped for ranged tho and hover blasts.
Intuition is great - +damage, +range, +control duration is pretty much 100% of what I want on my dominator, especially if you use cones with any frequency. Seeds of Confusion + range = <3
What I pick for dominators varies by the build and powers.
For Mind/Fire, I would absolutely permadom that sucker with IOs alone, then slap Musculature on him/her to crank out that damage. In my experience, domination alone needs no additional buffs from an Alpha. If you're permadom, you're good to go on control.
For a Plant/Psy, it depends heavily on your build. If you need additional recharge for Drain Psyche, then Agility or Spiritual. For more damage (and added -Def), Musculature or Radial Paragon Intuition.
Too many alts to list.
What's the general agreed upon alpha to go with on a dom? in my case specifically a mind/fire and a plant/psi