I get this question a lot, and my answer is always the same, "the best you can afford."
This is a question I get a lot, and a tough one. It really depends on A) what you're trying to do and B) what your budget is. If you think you're going to delve deeper into photographer and try to get serious, I'd go ahead and look at beginning to start building your kit (by say purchasing a fast prime lens or two). If you don't plan to really pursue photography as a serious hobby or even a career, but still want to take some nice pictures, buy a nice mid-range zoom. It will probably cover most, if not all, of your needs.
If you do it right, your camera gear should last you for years. Think of purchasing good gear as an investment. Do you plan on earning some kind of return on that investment?
For instance, I bought my first speedlight when I starting actually making money from photography college. It was $350 and was pretty much all the money I had. I've been using that same speedlight for nearly a decade now, and it has paid for itself many, many, many times over. At the time, it was a tough call, but I looked at the bigger picture in terms of how I wanted to build my kit (I primarily use speedlights to shoot on-location gigs), it made sense. My Nikon 70-200mm f2.8 lens was the same way; it was a huge investment when I got it, but I was shooting a lot of sports, and it's paid for itself over the years and is still my go-to lens for either sports or candid portraits.
Here's one thing to keep in mind; a good lens will make images from an "okay" camera body look good, but a cheap lens can make a great body look terrible.
You can never go long with picking up a fast prime lens. A 35mm or 50mm f1.8 is a good, not-too-expensive investment, and it's a lens that you'll probably use for years. I mention the 35 and the 50 because on a cropped-frame sensor, a 35mm has roughly the same field-of-view as a 50mm on a full-frame sensor. A 50mm lens (or one that functions like one) is a good starting place to build a kit. With a little backing up or getting closer, you can shoot just about anything with it.
If you're going to go with a few primes, look at the 28, 35, 50, 85, and 105 (all f1.8, don't spend the money for a 1.4 or even a 1.2; you're not going to see a big enough difference to warrant such a large jump in price).
If you only had one lens and it wasn't going to be a prime, I highly suggest something like a 24mm-70mm f2.8. You can do a variable aperture lens (the ones that list their aperture as something like f3.5-5.6, but you'll just wind up buying a fixed-aperture lens later on down the road, because you'll get tired of the loss of light when you zoom in).
To give you an idea, when I first started out with a DSLR, my kit included a 18-55mm f3.5-5.6, and a 70-300 f4-5.6.
I was given a 50 f1.4 a little later on, and after shooting with that, I really began to understand the difference between variable aperture zooms and "fast glass."
Here's the TL;DR version.