I would suggest something with manual controls that emulates higher end cameras and makes it easy to use. Lots of low end SLRs only have one command dial, and usually by default, it operates the shutter speed. If you want to manually change aperture, you have press a button, hold it, then move the command dial. Sounds trivial, but it's an extra barrier to make you not want to try manual control.
You don't always have to use manual, but when learning basic exposure theory, it's a good thing to do.
Higher end cameras have a separate command dial for both, so you can adjust speed and aperture quickly without taking your eye from the viewfinder.
For this reason I would say get a used Nikon D80 or D90. They have a two dial system. Not sure if any lower end Canons do... but others will advise.
D80s go cheap used on Ebay, and they're still an excellent camera.