Black Steed. Because i ran all way from Stormwind to Menethil Harbor to buy it as my first mount when i was level 42 back in Vanilla (2006). I grinded 2 levels killing mobs for the gold i needed for it. I still use that mount as ground mount (thanks cataclysm i can use it as %100) as i still play this very same character after more than 5 years.
Nowadays it's an overwhelmingly common mount, but my favorite mount in the history of playing my main has been the Winterspring Wintersaber...I know, I know, it doesn't have the rarity value it used too, but I just love the looks a) of nighsaber movement and b) of this particular nightsaber in shadowform