Assuming this plague only affects domesticated pets, and I can just go out and hunt down/recapture rare wild pets... it's still a tough choice!
For me, based on (non-wild) pets that have served me well throughout my pet battling adventures, it'd be a toss up between Anubisath Idol
, Darkmoon Zeppelin
, and Mechanical Pandaren Dragonling
Hmm, I'm probably missing a couple of non-wild pets that deserve mentioning (maybe fit in a moth or two in there lol) but gah. So many to choose from!