Anything much more than 15ish, and it gets a lot harder to keep the animal in your scope after the shot. I personally prefer 15 or 18 on the high end, and typically shoot at somewhere around 10-12x when I just dial magnification for what seems best for the shot, without actually looking at what power I'm dialing to. And, I have very accomplished shooters and hunters on here telling me 9 or 10 is absolutely all that you need. Anything more than 15-18 is genuinely overkill.
What matters most is a reliable, durable scope with a reticle that's fast and usable in the widest set of field conditions, for distances you are personally capable of shooting to right now. Don't get high on internet scopium, you won't shoot better with Tactical Timmy's Sniper Scope. Everything else is secondary to dependability and a good, useful reticle.