What lens, for you, is the most "fun" to use? Not necessarily the one you use the most, the sharpest, the fastest, etc, but the most fun? Bryan wrote of the 100mm f/2.8 Macro (not the new L version):
"
The Canon EF 100mm f/2.8 USM Macro Lens is an excellent macro lens - and may be Canon's most fun per dollar lens."
I enjoy macro photography, especially of nature--so much, in fact, that I got a Sigma 180mm f/3.5 Macro lens in addition to the Canon 100mm f/2.8 macro. (The Sigma lens may not be as good as the Canon 180mm f/3.5L--though some contend that the Sigma lens is as good or better--but it's a lot less expensive: $699 vs $1450 new at B&H. (I paid $526 on eBay, including shipping.) The Canon lens would have to be a lot better to justify double the price.
On the other hand, a super-wide-angle lens (16mm full frame, 10mm on 1.6x) can be fun, too, not to do wide landscapes (usually deathly boring), but to get really, really close to the subject. (It's different from a macro lens, as the super-wide-angle has a low magnification factor.) That can result in interesting perspective. Objects look very round, rather than flat, for example. Several times, I've given myself an assignment to use only my Sigma 10-20mm--though I may cheat and mount the 100mm macro some times.
I guess that means that my 100mm macro & Sigma 10-20mm are my most fun lenses. What are yours?