From my experience in the campaign (Brutal) my favorite abilities are the super Chrono, the 3 laser attack from the Spear of Adun, the passive that allows your units to survive longer, Time Stop is brokenly OP in some missions.

As for the units:
-Zealots- a tie between the Nerazim one and the Templar one. I like the AOE, but Nerazim on few cases are extremely useful.
-Stalker- Stalker. Until more dominating units can enter the field, microed blink stalkers are the way to go.
-Immortal- I used the one with the barrage ability, won me a few missions. I read that the Taldarim version is better, didn't experiment much with it.
-Dt- I liked the Templar version the most, the one that attacks so many times. The one that disables seem bugged.
-Templar- didn't use much, used more Dark Archons on few Zerg missions.
-Colossi- The purifier variation. Reavers are not so good, Taldarim ones? I want AOE, sry.
-Voidrays- the red version- bloody OP, AOE Bronze League tactics ftw.
-Phoenix- didn't use except Corsairs for the last mission, combined with DT's worth their cost in gold.
-Carrier- for all but the last mission I used the repair version, fully repaired Voidray fleet? I feel sorry for the enemies. In the last mission I used the Mommaship, I needed that vortex in the last few minutes.
-Sentry- tie between Taldarim and Purifiers.