Mine is the Skyracer Pegasus because it reminds me of Wysteria and it's really pretty!
I would have to say the surfboard you get from Islanders Hoard Pack. For one thing, I don't even HAVE this mount yet, but I plan on getting it soon. Though, if I were to choose one right now that I have, it would have to be the bone dragon for dragons hoard pack :).
My favorite mount would probably be the Crockagator or Bengal Tiger. The Crockagator does a cute little hip motion when it moves and looks pretty cool too. I love the Bengal Tiger because it isn't too big and bulky like the mount that comes with the Majestic bundle. Also it runs smooth and is easy to maneuver on streets littered with monsters.
Actually you CAN get this mount from a reward from Grub Gaurdian! Specific Grub Gaurdian maps (like Wysteria) drop packs, including the Dragons Hoard Pack! So, though it is not exactly attainable "in game", you can still get it from Grub Gaurdian, or G.G. For short. :)