I've seen a lot of competitive armies take an inquisitor with servo skulls. But then, if you're running more imperium stuff...
I like to run little 18-point units: 1 psyker, 2 bare bones acolytes. Stick them in cover and use divination powers on other imperium units, or give them telepathy for that 1 in 6 chance of invisibility plus threat of psychic shriek if the enemy gets too close.
And you might want to give the inquisitor rad &/or psychotroke grenades. I stick him in a blob of guard for Ld10 orders and stubborn.
Or another option: Get one with terminator armor and a couple ministorum priests and enjoy the rerollable 2+ saves in the assault.