I already answered Siege Tanks, but Ill Elaborate on some close seconds.
Naturally, units that are powerful in the Multiplayer portion are very nice to have. Great single player, but Multiplayer is the meat. So I gotta say that the vastly improved utility of infantry from the addition of the Medic is unmatch...wait
Okay, well, the cheap anti-air and versatility of the Goliath is...hmm
Well at least for anti air, there's the Wraith which...
Okay, for the ultimate in anti zerg rush, the Firebat...
Okay, the ability to heal vehicles using the Science vessel just...
You know the defensive capabilities granted by the Perdition Turr...
I love how the Spectors are able to...
Man, harassing with the Diamondback is so...
Congrats Blizard. I could for some reason vow to never play the Protoss or Zerg ever, and I would still be anxious for the release of the next 2 installments.
(If anyone's a fan of the Vulture, I would like to say that I am looking forward to the Vulture assuming it has an upgrade that allows the replenishment of Spider Mines.)