HQ choice all depends on your armies play style. The chosen HQ will then be able to use his special rules or fighting skills to for-fill the role you've given him.
For me personally I take Grimaldus and a single servitor, chuck them in a landraider crusader along with 10 marines and 3 scouts. They ride up as fast as possible into the enemy's front lines and then when the chance strikes, jump out and start killing as much infantry as they can find (Example: Fire warriors, Guardsmen, Tactical Marines) The Zealot bubble and FNP bubble from them give a lovely boost to the unit.
I also run a second LRC with another unit of 10 crusaders and 3 Neophytes that charge up alongside Grimaldus's unit and raider. Using the 6" bubble from both Grimaldus and the servitor means I hit the front lines with Grimaldus, 20 Marines, 6 scouts and a servitor all who get Zealot and FNP and if a scout dies from over-watch then laugh as you gain rage and counter attack at the same time.