I was going to say my 17-40L as it's my workhorse and delivers great images everytime (unless I bung it up!) but for sheer fun and joy with some killer images I'll say my 100 F/2.8 Macro.
Soon I'll be buying one of the five 70-200 zooms and I suspect that will take the prize soon enough. I'm quite happy with all of my lens choices really, even the 50.
On a side note (a little off the topic) I had put off buying a speedlight for quite some time because I thought lenses were more fun and I'd get more satisfaction, but my 430ex has really brought new life to my photography and it's a blast playing with it.


