This is a hard question to answer in some ways, but at the end of the day, my favourite lens is also my most used lens. For the past few years, the lens that has held this title in my lens arsenal is the Canon 15-85mm. Great image quality (sharp, contrasty, good colours) - fast, accurate USM AF and 4-stop IS.
I just find it a very godo combination-package - a quality walk-around, that can often meet most of my photography needs in the average 'day-out' shooting. 24mm - 136mm is a great focal length, and it doesn't really have any IQ failings. I take this lens when I'm capturing the official photos for camps and other events that I lead.
In terms of other lenses, I see all of them as more 'specialist' lenses, and for particular circumstances, they 'are needed' / my favourite.
eg: Sigma 10-20mm f/4-5.6 HSM EX (ultra wide angle, particularly for landscape, sometimes for architecture)
Canon EF 100mm f/2.8 USM Macro (for close-up photos of flowers, plants, insects, details - and even occasionally ad-hoc portrait)
Canon EF 70-300mm L f/4-5.6 IS USM - my wildlife and birding lens - for BIF as well as the outdoor portrait