There's no one lens that will meet all your needs. Also, you don't mention budget.
Wildlife usually means a long lens. This could be anything from the long end of a moderately priced consumer zoom to something like a fast 500 or 600mm.
Landscape photography encompases everything from ultrawide to ultra long, so that also is tough to answer intellegently without knowing the type of shooting that you will be doing.
Why not start off with something in the 70-200mm range, with the exact make and model depending on your finances. As with most things in life, the best is expensive, but budget lenses can sometimes be very good.
Do some more reading, and look around the Net and in magazines at the type of photography that you envision yourself doing and enjoying most, and see what focal lengths were used.
Like I said, an almost impossible question for anyone to answer but yourself.