Perhaps my fav lens at the moment is my Canon 70-300mm L f/4-5.6 USM IS. I live in Australia, and its captured a lot of photos in the 2 years since I've had it!
It's great on my 7D, from wildlife to birds in flight to candid portrait to even a bit of motorsports and/or field sports in daylight. The sharpness, contrast, colours and bokeh are very good - that is: very high imagine quality (IQ). Plus it's very portable and for me an awesome hand-holdable lens (IS helps!) up to the equivalent of 480mm, with snappy USM AF.
My 'fav workhorse' lens is the Canon 15-85mm f/4-5.6 USM IS, which gets the most use. Great range, very good IQ, and USM / IS as bonuses. Then I have other lenses that I consider more 'specialist' eg Sigma 10-20mm EX HSM and Canon 100mm macro USM. My 'next' lens will be a fast prime, probably the next Canon 50mm (with true USM?). The 'favourite' lens I would like to TRY is the new Canon 200-400mm L f/4 USM IS 1.4x... though I doubt I'll buy it (mainly due to size, weight and alleged cost, even though I can afford it..)