or
are my nominations for Most Valuable Utility.
You must have seen them.
There are more Yaks than Mammoths around these days, but they both do the most important job - providing a vendor and repair wherever you are.
Especially, as a relatively new player, if you are farming old raids to get a starting stake of gold, as described here:
it is SO convenient just to step out (sometimes you don’t even have to step out) and fence your ill-gotten goods!
Considering that you can get 20K gold from one run through Draenor, the Mammoth will pay for itself in time saved immediately.
The Yak is much pricier, but also has Transmog on board, if that’s important to you.
What else?
Do you have water-walking on your mounts? It’s worth getting that for the early part of the new expansion, when we won’t be flying.