Mine is the Obsidian Worldbreaker. Been wanting a Deathwing mount since I saw the Cataclysm commercial on TV that started my WoW journey.
Infinite Timereaver is a very close second, as I farmed that one like a nolifer, only for it drop when I stopped giving a damn. They always drop when you don’t try!
Share yours 
Mine is Rusted or Iron proto-drake (Achievements from Ulduar raid)
Second closest is Hounds of Sargeras mounts
Third one is azure drake
Rivendare’s Deathcharger. Dropped for me the very first run I decided to farm it. Prior to this I had only been in there for level purposes.
Definitely the headless horseman mount, dunno why it just feels “special” for some reason 
On my hunter, Garn.
On my DK the Mawsworn charger, it just screams DK
It’s a rat.
I’m a blood elf in the heart 
Flying Mount: Ashes of Alar, Cloud serpents, Sundancer, Blanchy.
Ground mounts: Hawkstriders
Sea mounts: Sea horses
The golden Legion Paladin’s Horse. I love the way it has no weird wings and just gallops through the sky, fits perfectly with my Pally.
Special mentions for Invincible that has a special place in my heart (because of the lore behind it) and Anzu.
It’s the Sylverian Dreamer. I also used the Iron Skyreaver and Mystic Runesaber a lot in the past.
Idk how I feel about all of them being store mounts.
Ok, I’ll give you one that you actually get from playing and that’s the Frostbrood Proto-Wyrm. Really digging it.
Zulian Tiger.
I like bird mounts using the Anzu model. Like the Corrupted Dreadwing from Tanaan WoD and the undead one using the same model from Maldraxxus. Do not have either atm, but plan on getting them at some point.
Priestess’ Moonsaber
The coalfist gronnling just because it looks awesome😁
I think the larion flyin mount. Or hh but im bored to use it
those 2 also fly
“Ancient mounts of the Highborne, now left to run wild in the Broken Isles, they can rarely be found due to their latent magical powers.”
It looks incredibly detailed, especially for a Cataclysm mount.
Spectral Gryphon and Tyrael’s Charger
I’m a simple clown
Any cloud serpent.
and Hogrus…coz who doesnt love a flying pig!?