To fit which camera?
If for a Canon, the EF 400mm f2.8 L IS is supposed to be pretty good, though I haven't tried one myself. Don't know about other makes.
and is also the heaviest lens Canon make....bar none

Ah, but he asked for the best lens, not the lightest.![]()
i wanted to find out what the best lens would be for wild life and nature?
thank you please feel free to comment and i would really need the help