Which lens you use the most, and which one gives you the most fun?

Well, as my work photography differs from my personal-time shooting, I will differentiate beween the two. At work, on 7D cameras, I use a 100mm Macro L for more scenes, but find the 10-22mm EF-S more fun. I shoot evidentiary photography, at night, while wearing a police badge, and these two lenses do just about everything I need.

On my own time, nothing is quite as much fun as my EF 40mm 2.8 STM, usually on a 5D, but also fun on a cropped-frame camera. This is largely due to the 40mm field-of-view being very natural, and the tiny size of this lens, which transforms a heavy DSLR into an amazingly handy, nimble camera that can be carried in a quite compact belt pouch. I should mention that the 10-22mm is very fun to use during my personal time, too. (I own the photography equipement I use at work.)