The pack mule. I think they’re cute.
Plus, I have this character in tier, but most of my character transmogs are of ‘every day outfits’, or as close to them as I can get (look like a farmer or clerk or very simple soldier, etc) so the mule works well with them.
Otherwise, for flying etc, I just try to find something that goes with their transmog.
My ROFLcopter.
On my druid the crow flight form… On my rogue my legion spec mount.
Priestess’ Moonsaber or the Cartel Gear-Glider.
The Red Flying Cloud is probably as close as I’ll ever get to having the Silver Surfer’s surfboard. If I could get that surfboard, call off the search, my forever mount is here.
The Flameward Hippogriff goes very well with my current set. That also goes for the Cliffside Wilderdrake, as I have it customized (with bronze scales).
Sunstone Panther. It can hold my Tauren and highly versatile in being a ground and air mount. (Until having to go to the Dragon Isles and having to switch to the Highland Drake.)
Hmm… don’t think I can just pick one, especially since I use mounts based on the race/class I’m playing. Overall, though, I would say my top three are:
Swift spectral tiger. I farmed the gold. Didn’t use my CC. Actually didn’t even have a credit card at the time. And no way I had hundreds of dollars to spend on it in Cata.
Very jealous. Thousands upon thousands of attempts and nothing to show. Think it’s one of best looking mounts.
Chilibean loves his polarbear.
on this character, it’s flying carpets; and the black stallion for when grounded
most of my alts utilize the flying cloud when solo, or else any passenger type for when i am dual boxing outside of the dragon isles
Current mount is the Kitteh, it’s a good ground a sky mount.
But normally, this has been my goto flying mount for Piddy:
Her ground mount has been:
I also particularly liked this one from BfA, it’s my DKs main ground mount:
For my DKs flying mounts, I use the Proto drakes: red for blood, blue for frost, still trying to get the green one for Undead.
My Druid doesn’t have one, my Warrior isn’t really set on anything. I think he’s using a raptor. He’s ambivalent.
Fallen Charger … (which should be able to fly).
This has been my favorite for years:
Bloodfang widow. Wanted it since it first was offered. Saved and saved but finally bought two tokens. SOOO cool.
BC Netherdrakes
IDk what it is but the sheer mobility feels of this ground mount are amazing and it feels fast too!
it’s a good jumping mount.
Flying mounts I like medium/small
Dragonflight it’s
Given to me by the Dark Lady herself. 
Plagued proto drake… If I had one. Getting mounts off the bmah but not the one I want. Got jipped out of it back in Wrath was 3 achievements short 
Out of the one I do have, Time-Lost Proto drake.
normally I’d be right there with you, but I don’t find that for a mount called “white polar” it’s particularly white!
I think it would be one of my favourite mounts if it were a somewhat brighter white.
Currently, Divine Kiss of Ohn’ahra is my favorite mount, but I can’t use it because of dragon swimming.