I'd say 17-55/2.8 IS, or 15-85/3.5-5.6 IS if you don't need f/2.8 (and with two primes in the range, you may not). Both will give you L-series optical quality with a much more useful focal range on APS-C, and both will actually give you as good or better IQ than the 24-105L, and better than the 17-40L, when used on your camera.