My favourite lens is...

I took this HDR photo of a dead tree in Masuria from 5 exposures at 2 EV spacing.

And the answer is: Canon 24-105 f/4 L IS USM. I used it for as many as 21% of my already developed photos since I started photographing with digital cameras back in 2008.

I already mentioned it a few times but I will repeat it once more: it's a really great lens and very universal one too. At the time I was buying it I was also considering Canon 24-70 f/2.8 but longer focal range turned out to be really helpful in a number of situations. If I go shooting with only one body with a single lens most of the time this is the lens I take with me. It allows me to take both wide-angle landscape shots and some nice close-ups as well (70 mm sometimes wouldn't suffice). What's more if you use a crop camera like for instance 7D, the focal range becomes 38-168 so you can have an "entry level" tele-zoom lens if you cannot afford 70-200, 70-300 or similar.

The lens is quite compact and its weight isn't too heavy. It suits 5D MK II perfectly (it was/is its "kit" lens) but I had no troubles using it with other cameras like 50D and 400D as well.

Canon 24-105 isn't ideal though. Two biggest issues for me are chromatic aberration and flares. Both are much worse than in Canon 24 f/1.4 L II USM but given that the latter is a one fantastic prime - it is to be expected. But you know when you get used to images which contains very little of them produced by Canon 24 mm prime it's really difficult to stop comparing the results.

Sharpness is good - it's all that I can say :) Above photo was taken with this lens and it should appear pretty sharp (at least it does on my monitor at the time of writing). I've definitely seen sharper lenses (especially Canon 100 L Macro and Canon 70-300 L lenses) but sharpness is good and with a little of sharpening in post the images can be made very sharp.

Lens could be a little faster but as I use it mainly for travel photography it's sufficient most of the time - especially as from my tests this lens is sharpest between f/7.1 and f/8.0 and these and slower apertures are the ones I use most of the time with any lens.

So summing up this very short post - it's a great universal lens that all travel photographers should have with them. If you want to travel light it is perfect choice.