Canada Lynx or Saber-toothed Tiger

Given that mankind is indisputably the most destructive organism ever known to exist, anywhere in the cosmos, I would lean (not entirely tongue-in-cheek) toward reviving some ancient carnivore that breeds prolifically, and has an insatiable appetite for human flesh. We need some competition!

Seriously, though: Unless the candidate species for revival would serve a clear ecological purpose, I think we should invest our time and energies preserving today’s threatened and endangered species, rather than dwelling on the past. Better to save the Canada lynx than to resurrect a saber-toothed tiger.