Hmmm, some of my favorite things to enslave.
The felguards at the forge camps in Nagrand. Nice being able to solo almost every group quest in Nagrand with one of these guys.
The tinkerer guys in Mech, they are soooo nasty.
The imps from Kara are kinda fun to play around with, thinking about it, I dont ever think ive tried to enslave one of the succubus type mobs in the trash leading up to Maiden, may be something I'll have to try.
Staple favorite of mine though has to be any of the Evil Eye type demons(the ones at Deaths Door, or in Netherstorm). They have a nasty little debuff they can put on mobs, it doesnt last but a couple seconds, and the ability has a couple second cool down, but it increases all magic dmg done to the target by 50%! Huge Crits! You have to get used to timing using the ability to apply the debuff right before the spell hits the target, but once you get used to it its amazing. I have been meaning to play around with one since I respeced DT/Ruin for Vashj, Demonic Knowledge and SL, should be interesting to see what kinda numbers I can get.