My own choice on an APS-C body is, without hesitation, the 17-55/2.8 IS. But that suits
my preferences. It may not suit yours. I also see no harm in buying L glass for crop bodies, if they have the specs you want. I bought a 70-200/2.8L IS and 100-400 when I had only a 30D. They have continued to serve me well with my 40D, 50D, 7D, 1D3 and 5D2. An investment in good glass is rarely a waste. It will last for years and probably outlast several generations of body. That said, the 17-55 is an excellent lens and in my opinion there is no L glass that competes on equal terms. Indeed I was really disapointed when I bought my 1D3 and felt "forced" to buy a 16-35/2.8 and 24-70/2.8 to cover the range covered by my 17-55.
I would be very cautious about restricting myself to 24mm (38mm equivalent) at the wide end. That's worse than pretty much all point and shoot cameras, many of which are now trumpeting wide angle equivalents of around 24mm to 28mm. I'm also not a big fan of slow or variable max apertures. That cuts out f/4 lenses like the 24-105 and lenses such as the 15-85 for me.
But quite honestly, you need to satisfy
your requirements, not mine. A walkabout lens for wildlife might be the 100-400. A walkabout lens for landscapes might be the 10-22. A general purpose outdoor lens might well be the 24-105, but if you want to shoot indoors too the 17-55 and its f/2.8 aperture will probably be better. If you aren't fussed about speed or IQ then maybe a superzoom will be the right walkaround lens for you. You need to figure out the focal length range you want, the aperture range you want, whether or not you want IS, and then see which lenses fit your needs. Then you can see which of those has the IQ you desire.
Here's a big thread over on POTN covering the same question-
http://photography-on-the.net/forum/showthread.php?t=819629. I haven't read it but hopefully it will offer some insight.