Green Proto-Drake - it might seem like nothing compared to newer and cooler looking mounts but for me it meant I could get rid of the ugly wind rider. I got my reputation up with the oracles in order to buy the egg, but it took me about 6 eggs I think before the mount actually dropped. When I clicked on the egg and expected it to be aged yolk yet again and instead was the drake my heart leapt.
I can't remember the details now but on the day it finally hatched I'd had a rubbish day in WoW, probably a failed PuG or something. I soon cheered up!
(Deepstone Oil for the win)