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 absolute favorite is the Charity Fa-la-llama I got at the Christmas special a couple years ago. It glides and moves wonderfully, is not too big, and it is way fun to jump.
After that, the surfboard mount is also quite fun.
the "mighty steed" from this year's April Fool's special is hilarious. Being a long-time Monty Python fan, I usually get a good chuckle when I use this.
Other than those, I like:
Nightmare Kirin Treants (awkward at times, but fun) Pegasus-type mounts skyvern at times (fun to fly, but not really practical)
and the old standard enchanted broom is by far the most practical -- glides straight and fast, easy to maneuver.
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. :)
I would have to say that the surfboard mount from the Islanders Hoard Pack is my favorite mount. Though I do not have this mount yet, I plan on attaining one in the near future.
P.S. I think the full name is the Tiki Waverider, but I'm not sure...