Lady Vox  

As said above, Vox and Nagafen both have 32,000 hp. Nagafen can triple for about 225-255 while vox is around 200. Personally, I'd have to say Nagafen is a lot harder.
Nagafen: 32,000 hp, triple for 255, AoE spell Lava Breath ( 700-800 damage ), and what makes him the hardest is that friggin Mass Fear spell DragonRoar.

Vox:32,000 hp, around 200 a hit, AoE 500 damage ice spell. Yes she can Complete Heal, BUT it's easy to avoid unless your casters are oom. Have one full group of casters start a chain stun sequence when vox is at about 30% health before she starts healing. And whoever said she can't be stunned anymore either doesn't know what's goin on or has some really horrible casters. Trick is to lower resists first, easy to chain stun around the end. Have your group of casters stun, while about 5 or 6 wizzies Ice Commet the *****.
Nagafen is harder cuz of that mass fear spell, sends everbody running while nagafen can just summon you all back and lava breath you all to death and claw you for 225 until you're nothing but bloody pulp.

SCimitar of the mistwalker as well, effect of summonmistwalker, pet with 1hp, hits for like 146 i believe, duel wields.

at my experience Vox is harder than naggy tell you why , she can complete heal herself but thats not the worst thing , in the latest patch she CANNOT be stunned anymore..ouch..so you CANT interrupt her complete healing she cast a 500dmg AE effect and hit for 211dmg tripple attack can mass fear and summon you.
Naggy now: AE spell of 800dmg..kinda hard on balls.can summon you mass fear and hit for troppplie attack to 255dmg
worst thing about it is that when he fear you you may fell in lava ..kinda annoying paying a necro 500pp for summon corpse .
both of them got nearly 30.000 hp..when vox complete heal her self she is most of time under half of hp so..have to do it again and at that poir you should be a bit low on mana...

kill her and then give me a tell so i can go there and loot her =)