Canon EF 24-105mm f/4 USM L IS
This is my always-on lens. In the photo-bag it is always armed with flower-hood, attached to the body, ready for shooting. Not much to say - a solid performer in both optical and mechanical terms. It's not a most recent construction and the rather wide zoom factor involves some compromises, but given the flexibility, price and weight, there's not much reason to go for the optically superior models.