Nemesis: Hunter - Hunted

Alliance should select Blood Elf.
Horde should select Human.
http://www.warcraftrealms.com/census.php

Quest dialogues for all nemesis choices (Horde)

Human
A fine choice. Humans are a blight upon this world and must be destroyed or driven out of Draenor.

Dwarf
Dwarves are integral to the Alliance as they are the builders of war machines. For this reason alone they must be slain. Show no mercy, as none will be shown to you.

Gnome
Whether an aberration of nature or a cruel joke by the gods of your world, gnomes have no business living. Their extinction will clear the way for a more powerful species to rise. You must be the instrument of their demise!

Night Elf
Night elves, conspirators one and all. The night elf invaders must be crushed and sent back to their moon gods!

Draenei
Draenei...
<Raza'kul spits.>
You would do all the denizens of this world a great service by ridding these lands of draenei.

Worgen
These worgen that have invaded our lands share the same ferocity and animal instincts of the native saberon, but with none of their survival sense. Let their deaths serve as a lesson to the rest of this so called "Alliance".

Pandaren
These pandaren had a choice to serve in your mighty army, but instead opted to betray the Horde and join the enemy. Now they aid the draenei in their mission to kill all orcs. The traitorous pandaren of the Alliance must be exterminated.

Once you choose your "nemesis" you can't change back. I would suggest you pick a race that is well represented. Human I think would be the most solid choice for horde as they are probably the most represented race on alliance side.