Several Legion pets are just copies of previous pets or have slight differences.
Saying that, because of their appearance in the expansion, they may seem more easily available than the "classic" ones.
Among these, I'd tell that there are an H/P* [pet]Mudshell Conch[/pet], P/P [pet]Dream Whelpling[/pet], [pet]Stormborne Whelpling[/pet], [pet]Benax[/pet] and S/S [pet]Rose Taipan[/pet]. Stormborne is actually pretty unique in its own way, just the speed is a bit out of nowhere.
On the other hand, some pets provide something that there has been a lack or scarcity for quite a long. See [pet]Magma Rageling[/pet], S/S [pet]Wicked Soul[/pet] or H/P [pet]Blightbreath[/pet] for that, aswell as a novelty of [pet]Paradox Spirit [/pet] and [pet]Infinite Hatchling[/pet].
Bonus points for [pet]Bleakwater Jelly[/pet] and cousins for being the first aquatic pets that make a party on their own instead of being stalemates, pumpmasters or beast/critter wannabes.
There are also pets that may not be *the* best, but due to their exotic movesets they prove worthy during
Family Familiar progression. These are absolutely subjective and off the top but there belong [pet]Dreadmaw[/pet], [pet]Ash'ana[/pet] or [pet]Risen Saber Kitten[/pet].
EDIT: Thanks to @Aranesh for mentioning [pet]Broot[/pet]! Though still not as easy to get when the RNG's grasp is tight.
Also, the falcosaurs. Because [ability]Falcosaur Swarm[/ability]! is OP. And they provide mounts [url=http://www.wowhead.com/falcosaur-guide]if you follow them[/url] for two weeks.
* [url=https://www.warcraftpets.com/wow-pet-battles/breeds/]Wʜᴀᴛ ᴅᴏ ᴛʜᴇsᴇ "H/P" ᴇᴛᴄ. ᴍᴇᴀɴ?[/url]