Canon 70-200mm f/2.8L IS II USM
The Canon 70-200mm f/2.8 is one of, if not the most used lens in photography (well, maybe not this specific lens, but 70-200mm lenses in general). I use this lens for getting close up shots of performers and detail shots of instruments, stage decor, and any other things that stand out to me as interesting details. This one stays on a camera most of the time while I might switch between wide angle lenses on my second body.
For Nikon Shooters: Nikon 70-200mm f/2.8G ED VR II