Hum: Anub Idol, for obvious reasons, with 333 fiend imp as close second
Beast: Any Direhorn, p/p or p/s preffered, since it's so broken
Critter: Any s/s bunny, because critters aren't really viable atm in pvp anyway, and this guy is crucial for thundering elemental
Magic: Jade Ooze p/p, fun to double dot something and drain life back, until Sandstorm comes up
Aqua: Puffer, Surge, Heal, Pump on 325 power pet is fantastic load-out
Drg: Chrom, staple to kill BoFs in 5 turns. Would love to use Ony Whelp from FoS, but Humanoids and Darkness teams just slaughter him (and lil' Deathwing too :/)
Flying: Possibly p/p Yellow Moth. Raven/Crow would be staple in Darkness team but I can't seem to make it work.
Ele: Sandling after they buff racial, Lil' raggy otherwise
Mech: Son of Animus, still waiting for Daughter of Animus with same moveset but s/s breed to make any use of second tierF
And for Undead, I have no idea. Maybe Valk. But there is soooo many great pets. Bone Spider, Mr. Bigglesworth, Scourged Whelp, Ghostly Skull, Shadeling, Crawling Craw...