That's a very hard question. The reason I have a DSLR in the first place is so that I can change lenses for the situation. I'm with Mark on the 24-105 in that it is indeed a fantastic do it all type of lens, but since I also do a lot of nature and railroad photography, I find myself using longer lenses more often than not. I guess if I had to pick just one I guess I would say my 100-400L(although it is currently broken). I already have the new version on pre-order and look forward to putting it through its paces on the new 7D2.
Sample using the 1DMK IV
4067 East making waves by
lennycarl08, on Flickr