Right now my favorite lens is the Canon L 70-200 f/2.8 IS. I like the fact that it has Image Stabilization. It has excellent image quality and the focal length is right for me when I"m out walking. It has excellent bokeh. Down side, is the price and weight. I get a little tired if I'm lugging it around all day.
I just ordered the Canon 17-55 f/2.8 IS and it should be here tomorrow. I hope that it will become my favorite everyday lens.
I have the Canon 70-200 f/2.8 IS as well and I love it as well. It is sharp and versatile, I like the fixed 2.8 aperature as well. The IS is great for lower shutter speeds. Plus I can add a 1.4x teleconverter making my lens a 84-280mm f/4 and still retain very good image quality. My next purchase however will be a wide angle like Jimbo and Linda are referring to. I have the lens kit 18-55mm, but it's pretty bad. It really comes down to what you are going to be shooting with it. I bought the 70-200 with the main purpose of shooting hunt tests, hunting shots, and kids sporting events, one of which being hockey so the 2.8 aperature really comes in handy. You can save some money and get and F/4 lens and/or get he non-IS versions, but you do give up some versatitlity, I feel, when you do that.