If only one lens, I say a good versatile zoom, like the Canon or Sigma 24-105. Why? Because, first impressions are the most important, and composition is more into the first impression than, say CA in the corners! Nobody gonna care if your corners are sharp if they don't have a good first impression (composition) and rapidly click or walk on to the next shot.
There is a reason why my Dad (who was a pro who shot with a Rolleiflex 2 1/4" and medium format) got so excited when he got his first 35mm zoom in the early 60's, and why they have come to such prominence. It's that composition is more important (in the bigger scheme of things) than sharpness in the corners. If people have a good impression with their first glance, sure, they may come to inspect your corner sharpness, but if they never get that far because they moved on to the next pic, it doesn't much matter.
Just my $0.02 worth.