Karandras, is the best PL, there is not really much contest. He takes a unit of his own aspect and gets them where they should be.
Jain Zar, is probably number 2 as she grants the banshees great mobility, however they should not be running, they should be charging.
Asurmen is number 3 on my list, by manipulating the warlord traits for a chance of fate's messenger, and his invulnerable save he is the most durable PL, and he can bite back in melee.
Baharroth is the cheapest one his "flashbang" and hit&run is why he is quite useful, he also counts as the 6th model for the grenade pack.
Maugan Ra, while his new weapon is very cool, it's still not much more effective than 2 scatterbikes. Problem is he doesn't help his own aspect much, he brings nightgoggles and some daemon cursing.
Fuegan, while he has his multimelta and sort of a non-unwieldy powerfist, FNP allows him to stick around a little longer, but he's still not doing much.