There is a good reason why Canon's 100-400 f/4.5-5.6L lens, though nearly a decade old now, is still a perennial favourite with photographers – it's a terrific all purpose and wildlife lens. The Mark II version is in many respects even better.
Sharp; Easy to handhold; Well constructed; Terrific usability thanks to wide zoom ring
Expensive; f/5.6 maximum aperture may deter some