Mike,
What Canon lens did you try? I ask because, the kit lens that usually comes in a kit with a Canon camera is no gem. Probably no better than your sigma lenses. That's not to say they are bad lenses...but when compared to a great lens, you will see the difference. Of course, most great lenses will cost many times that of what your camera will cost.
I always suggest the 50mm F1.8 lens. It's not a zoom but it's optically very good, much better than those kit lenses. It's fast (large max. aperture) which is very nice to have...and best of all...it's very inexpensive.