This question posted on a forum recently got my blood boiling. Why ask people to answer such a dumb ass, ill thought out, question in the first place?
If he can only afford one lens the questioner should say so; if he wants to take broad vistas or flora in the landscape he should say so. Does the writer assume all landscape pictures are the same or that other people are mind readers? No, I think the writer of this question is just too idle to think it through and too inconsiderate of other members to provide the information they may need to provide a proper answer – if they are willing to give up their time and experience to help a stranger in the first place.
However for the benefit of those that need it answering my answer is; The best lens is the one you need to take the picture in question.