My favourite specialist at the moment is Zagara. I feel that she's a very strong all around hero that can do just about everything very well. She can lane with her minions, jungle, grant vision via creep and also regenerate and move faster on it. She can stay in combat due to Rapid Incubation and she has the possibility of aiding team fights greatly with Devouring Maw. Her damage can also be pretty scary and strong enough to push someone out of a fight if played well.
So, what is your favourite hero? I realize that Sylvanas isn't out yet but you can base whether you like her or not over the hype or the gameplay you've already seen.