Ythraks for the wind. Sure, they look kind of dumb, but they're mean bastards with a sonic lance. And they're huge, so your friends can ride along too!
Wyvern. It just looks the most impressive. It's like a dragon, but for the skyknights who want to just feed their mounts cows, not split the treasure with them.
I voted Chimera because, let's be honest... Griffin/Hippogryff is so typical, and I don't know what a Yrthak is. Chimera aren't the fanciest fliers, but they're messed up enough that you will command respect from anyone that sees you swooping in.
Had to go with manticore. Intelligent enough to reason with and develop tactics with, has a ranged weapon, and are wicked scary. (although I prefer the more classical man-like version)