Green marsh hopper on ground 
The Water Elemental from Jaina flying
Although a few of my characters would not use that flying mount I would still use the Mage and Death knight class mounts for them and my Druid will stay with flight form lol and if I ever get that stupid Fel reaver from Hellfire citadel I would use that for my Demon hunter.
But for ground it’s pretty much said and done all the way!