Only looking at Canon lenses, which I am a fan of, here are some helpful tools for lens shopping.
Canon's Lens Site
All 72 Canon Lenses, sorted from low-high:
When looking at medium-range zoom lenses, it would be really nice to have a "constant aperture" throughout, like these three lenses:
Canon 17-55mm f/2.8 IS
Canon 24-105mm f/4 IS
Canon 24-70mm f/2.8
Outside of those three lenses the medium-range zoom lens is a tricky game to play. I would avoid lenses that promise a wide range, such as 18-200mm. They will definitely not be able to pull it off without noticeable flaws on both ends of that spectrum.
I absolutely love my 24-70 f/2.8L. But, that is not THE lens, there isn't ONE lens to buy, it is all about building and planning your camera bag and your lineup for the future. For instance, here's my lineup in the case right now:
Tokina 12-24mm f/4
Canon 24-70mm f/2.8L
Canon 70-200mm f/2.8L IS
Canon 2x Extender
Canon 50mm f/1.4
See how with three lenses I can go right up the ladder? And with the 2x extender on the biggest lens, I can technically jump to a 400mm if I absolutely need to?
It's more about your personal setup than it is THE lens. Just plan ahead so that you don't have a 28-135mm and plan on adding wider and tighter lenses. At 28mm already, it would mean a fisheye like mine, but you wouldn't want to pull it out on a regular basis when you just need those extra 4mm. Or, on the telephoto end, you could have a 70-200mm, but your medium lens overlaps all the way to 135mm, halfway into your big lens.