well.....if your new and somewhat cash strapped and own a canon DSLR. I found my first walk around lens to be great. It was a canon ef 28-105 f/3.5-4.5 USM. Make sure its the 3.5-4.5 USM version, since they make a bad version in 4.5-5.6. I bought it used for 150 off of Fred miranda. Great lens, not real wide though. My second lens was a ef 50 1.8. Best lens for the money that I have seen. new for $80 at most online photostores like BHphoto.
If you have money to burn.....My favorite lens right now has been the Canon ef 24-70 f/2.8 L. I have been renting it, but it will be the next lens I buy. It works well with my wedding lens kit consisting of tokina 12-24, 70-200 2.8 IS L, and 50 f/1.8