The highest I've seen was a 2800 crit from Surge of Power from an Azure Whelp. That was against a flying mob, and with the 50% dragonkin extra damage boost. The dragonkin boost does activate on attacking backline pets with aoe effects and by killing pets, so you can often get it on the first strike on a full health pet.
Xu-fu under 50% hp - prowl > moonlight vs a flyer = 2200-ish non crit. i imagine if there were a way to get a double dmg debuff up, 6k+. typically i would go prowl > moonlight vs any crow, round 2 is a 1 shot every time.