All of them.
Sorry D: haha it's just that all of them are awesome, and function with different armies, so you may want to consider the things you like playing with:
Arkhan: he is a "mini" Nagash that can cast spells owned by other Death wizards, so naturally, you may want to play with 1 of every wizard you have (useful spells are Vampire Lord's Spirit Blight, Necromancer's VDM, Kemmler's Invigorate, Mannfred's Wind of Death, Isabella's Dark Seduction, etc). Plus his command ability is really meant for you to bring as mani wizards as you can, raining death and destruction from 24'' (most spells)
Mannfred: is actually funny to play, specially with his Gheistvor's fluffy power. Other than that, he is more of a leader than the other two Mortarchs since his Command Ability is an area of effect ability, goes really really well with a slow-advancing army, where you want to affect as many of your units as possible, making Grave Guard and Spirit Hosts much more useable to get those special abilities to trigger. The more "attacks" are influenced, the better, so he truly is the Lord of the little guys.
Neferata: is great on her own, but even greater to combo with highly armored units, such as Blood Knights + Dark Mist. if you usually go for Morghasts, Blood Knights, Ushabti, Necrosphinx and other goodies that get the most out of staying power, Neferata is your go-to lady, because she can increase the value of your big guys by quite a lot (and she can "solo" almost any Hero too with an astonishing 2+ to Hit with all her strikes, rend 1 and 2 with a decent damage output, plus Nagadron's claws. Creating a brand new Vampire Lord when she slays said hero is quite something too)
=D choose wisely!