I have quite a few mounts! Some of my favorites are:
Heavenly Onyx Cloud Serpent: I was super stoked when this thing dropped for me, I wanna say during WoD. I threw so many alts against those world bosses! I find the monochromatic design beautiful.
Ny’alotha Allseer: This thing is ultra creepy and I love that about it! Totally didn’t earn it through my own merit. =p
Phosphorescent Stone Drake: I read like three books of A Song of Ice and Fire while camping for Aeonaxx. I love the way you fight him when he actually does appear. (For those who don’t know, he flies all over the zone, it’s pretty intense.)
Kor’kron Juggernaut: At first, I was quite depressed because I was actually able to kill Mythic Garrosh through some degree of personal contribution to the group during the brief time between the introduction of Mythic and the start of WoD, but I just never won the roll. Later in WoD, a friend lent me a lot of gold so I could buy it off BMAH. I was so grateful.
There are probably a lot more. I’ve been less active in SH and DF, so I’m missing more mounts from those expansions, but over the years I struggled to collect so many and, consequently, those mounts are quite meaningful to me.
The mount I don’t have that I’m most hoping to get right now is the Shackled Urzal. I love how creepy it is. (Apparently, I like scary things. xD ) One of these days it’ll drop!