What size wildlife? Bears? Warblers? Insects?
Larger insects a favorite? Canon 300mm f/4L IS, it has near macro capacity of 0.33x magnification, just the thing for dragonflies and butterflies.
Mostly birds, especially birds in flight? Canon 400mm f/5.6L (no IS, but you don't need it, your shutter speed is high). Lightweight, snappy autofocus, tack sharp wide open. This is my long telephoto lens.
Mammals and birds? Canon 100-400mm f/4-5.6 L IS, gives you the extra reach for small birds but allows use for average and large mammals
Mostly mammals? Canon 70-300mm f/4-5.6 L IS, also a great travel lens.
Ultimate versatility for a strong-armed photographer? Sigma 50-500mm OS, aka the "Bigma".
Bears? I'd go for as long a focal length as possible
You can never have enough focal length for small birds. For other wildlife, 300mm ought to be fine.
Go forth and research.