It depends for me.
I haven't used my voidwalker since like.....well, I don't even remember.
I use my imp a lot for blood pact, felguard when I had it.
Sometimes I use the felhunter when there's a lot of casters (I can't think of an example now) for spell lock and such.
I like using the succubus sometimes, but I find I have a lot of trouble, even when marked, keeping people from breaking seduce, myself included. I also find she dies easily and I get irritated having to use up shards on her.
I also enslave a lot and it seems to work out so far. Haven't tried it in a heroic yet though, but it worked pretty nice in Mech.