The question is a tricky one to answer because there are a couple of factors to consider, such as how much money you can afford to spend and what your photographic style is.
If you shoot landscapes, then you want to buy a wide lens such as a 10-20mm or 17-40mm. Both Canon and Nikon offer a decent 18-55mm as one of their standard kit lens. Although the 18-55mm is a inexpensive one, they make very good images from my experience. Of course if money is not a major concern and you shoot Canon, you can go with either the EF 16-35mm or the 17-40mm L lens. On the Nikon side you have the 14-24mm which is an excellent, high quality lens that makes fantastic images.
For those that want to shoot wildlife or anything else that requires more reach, you can look at 70-200mm, which is an extremely popular zoom used by many professional photographers. At 70-200mm you have the most popular focal lengths in a single lens. Both Canon and Nikon make very high quality lenses in this range, as does Tamron if you are budget minded. This lens is also available in both a less expensive F/4 aperture and the faster, more expensive F/2.8. If you want the F/2.8 without spending $2,000-$3,000, then look at the Tamron, which runs around $1,000 and from every review I’ve read by pros say that it takes every bit as soon an image as the more costly ones made by Canon or Nikon. Another extremely popular Canon lens is the EF 28-135mm, which I shot this portrait of my father with at 135mm length.
Your bread and butter will be portraits, then you want to get a prime portrait lens. The most common lenses for portraits are the 50mm, also known as the Nifty Fifty, 85mm and 135mm. When it comes to Canon, there are three of each the 50mm and the 85mm to choose from and again, it mostly depends on how much you want to spend. The 50mm F/1.8 is inexpensive at around $100 and makes really good images, the 50mm F/1.4 USM is around $400 and is much better build quality and takes even better images. The third is the 50mm F/1.2 L which is very expensive but also makes the best images. In the 85mm, Canon makes the $500 F/1.8, the 85mm F/1.2 L original and the newer 85mm F/1.2 L Mark II. The L series lenses are expensive, running around $2,000 and believe it or not, the less expensive F/1.8 makes much better images. The original L series 85mm had lots of issues with color fringing and the Mark II edition has AF issues. The third and final really good portrait lens on the Canon side is the 135mm F/2 L. I mentioned the wide angle lenses earlier, this image was shot using my Canon EF 17-40mm F/4 L USM.
Regardless of what you are going to shoot primarily, it is always a good idea to have 3 or more lenses as you never know when you might need another focal length or prime. I love to shoot landscapes, but like most photographers, I know I need to shoot portraits as well as that is how most of us pay the bills. Since I need to do portraits as well, I own the 85mm F/1.8 USM and the 50mm F/1.4 USM. I am also planning to at some point down the road to buy the 135mm F/2 L so that I have a little more reach.
I hope this information helps you to make some choices on what you want to carry in your camera bag, now get out there and make some images!