What's so cool about My Starry Pony? First, it's full of stars. I mean, look at that. It's made of stars. Of course, this was a model technique made popular by Algalon, who's all about warning the Titans that we mortals have totally run amuck. But the design is absolutely awesome. It's made of stars!
What else is cool about My Starry Pony? You never have to buy another mount. It scales from level 1 all the way through level 80. (No way to know yet whether or not the mount will do whatever cool trick mounts do in Cataclysm.) The Celestial Steed will automatically become the context appropriate mount for your character, moving at the appropriate speed according to your level and training. It will even switch between being a normal ground mount for old Azeroth, or an airborne princess in Northrend. You will only need a single action bar button for your mount -- it'll fly or fall appropriate to the area you're in.
The last thing that's cool about it is that it shows up in your mailbox, even for character you've created. That makes it the single, easiest mount to get. Warning, though: you do still have to train the riding skill, just like you would have to with a normal, mundane, boring, not-full-of-stars mount.
While requiring the training skill means that the Steed doesn't vastly change gameplay, its one-size-fit-all nature definitely makes it the most convenient mount, especially since you won't ever need to spend gold on another mount. (At least, not until Cataclysm changes the rules again.)
All you have to do to get it is head over to the Blizzard Store, and pick one up for 25 bucks.
Did I mention it's full of stars?