I use the Headless Horseman mount. It looks cool and it flies. Also it's the only mount I have that does not stay vertical. So if your on a side hill or something you can stand there sticking out at funny angles. Second choice would be the Cenarian Hyppogryph from BC. It looks cool and has a lot of animations. Its always doing something, looking around, stretching, pecking at something.

I would like to get Invincible from the Lich King. And my show off mount is the violet proto drake. They are still pretty rare with the year long quests requirements.