50mm f/1.8. It is cheap but with little compromise on quality. You'll learn alot from 50mm because:
1. Its a prime lens, meaning you need to walk arnd instead of relying on zoom. You learn about perspective.
2. It has the same perspective as the eye. (try it, place one eye at the view finder and one looking outside normally) Meaning most photos you take are just crops of what your eyes see. If you want the photo to be good, YOU have to start seeing creatively. You learn about creative perspectives.
3. It has a wide aperture. Lets you do really funky things with depth of field. One of the best things with f/1,8 is for taking portraits especially when used with a DX. You learn to control Depth of field
4. It has a wide aperture. GREAT for night/low light photography. You don't learn much here, but you have the flexibility of shooting in previously challenging places.
5. Its optical quality is good. You learn that sharpness and colour rendition, while overrated, does play a part in making your photo stand out. The same photo taken with the 50mm and your kit lens with the same conditions will likely show you slightly different variations and chances are, the 50mm version will have a more professional feel to it. You learn about what the hell is IQ and why people fawn over it
6. Its light and cheap (relatively) You learn the value of getting the most bang out of your buck ;D
PM is you need other suggestions, hope that helps