TheTS-E 17mm f/4 L Tilt-Shift Lens, if you want to take perspective-corrected architectural photos. Or, theMP-E 65mm Macro Lens, if you want to take photos of flower parts at 5x magnification. Or, theEF-S 10-22mm f/3.5-4.5 USM Lens, if you want to take sweeping landscape photos.


What I'm trying to say is, the best second, third, etc., lens is determined primarily by the type of pictures you want to take (and your budget, of course). In my opinion, you're best bet is to get the best quality lenses you can - a good lens is an investment that will last through multiple bodies.


The lens you have covers a very broad focal length range, which is very convenient, but it suffers from multiple types of distortions across the focal range, and has a variable aperture, letting in less light as you zoom in, meaning you lose the ability to stop action.


One recommendation is to spend some time with the lens you have, find a focal length you like, and get a fast prime at that focal length. TheEF 50mm f/1.4 USM Lensand theEF 85mm f/1.8 USM Lensboth offer good-to-excellent image quality, fast apertures, quick and silent focusing, and cost less than $400. If you just want to try out a fast prime, the 'nifty-fifty' (EF 50mm f/1.8 II Lens) is less than $100 - it's very sharp, although cheaply-built.


Alternatively, if you find yourself always in the low end of the range of your current lens but want better image quality, consider theEF-S 17-55mm f/2.8 IS USM Lens. If you find yourself frequently shootingin the upper end of your current zoom range, consider one of the EF 70-200mm L-series zoom lenses. If you find yourself at 200mm and wanting more reach, consider theEF 300mm f/4.0 L IS USM Lens.


Finally, don't just focus (pun intended) on lenses. Unless you already have one, consider an external flash like theSpeedlite 430EX II Flashor theSpeedlite 580EX II Flashto replace the horrible lighting provided by the built-in pop-up flash. You'll also want a good quality tripod (Manfrotto, or Gitzo if you can afford it). You may benefit more from one of those accessories than from an additional lens at this point.