IMHO, in this order; Karrandras, Fuegan, Asurmen, the rest of the Lords. Karrandras can literally tear shit up! Fuegan is very hard to kill and cuts through tanks like a hot knife through butter. Asurmen is THE anti-character...character lolz. His invul and his super-diresword can give even Yarrick the shakes.
I think the best one for its points is Jain Zar with Howling Banshees in a Wave Serpent, saying that I've never actually used her. . .
"Playing Warhammer is just like making love. It's usually done on a table and afterwards you always feel great shame. Plus like most fun things the older you get the less fun it becomes. So remember, always wear a condom while playing Warhammer - I've got mine on now."
Maugan Ra is my favourite. Best shooting and still awesome in combat. He doesn't really go well with reapers I wouldn't think. He'd be best with dire avengers. But, since that's not possible putting him with guardians could work.
Baharoth seems the most pointless frankly. At least with respect to putting him with his aspect. Hawks don't go into combat with anything that's not a vehicle and his gun is not good enough to rely on that alone to make him worth taking.
I've used Asurmen and Jain Zar, they have both performed admirably in their games. Asurmen is really tough due to his T4, 2+ Save and 4+ invulnerable. Zar is great on the charge with 5 WS7 and S7 attacks.