It's not light, it's not small and it's certainly not cheap, but it is the best lens I've ever used. It's the LEICA VARIO-ELMARIT - SL 1:2.8-4 / 24-90 ASPH.
For the above reasons I had decided that I would probably not get this lens and consequently used a lot of alternative lenses, via adapters on my Leica SL (Typ 601) camera. And the Canon, Leica R and Nikon lenses I've used have produced excellent results. However I have always wondered if I was getting the best out of my SL and it's excellent sensor. I also made the 'mistake?' of looking at some samples with this lens, which were seriously impressive. I was slowly coming to the decision to get one and try it out. A few weeks ago I saw one at a good S/H price from a reputable dealer so I decided to bite the bullet, warm up my credit card and go down the gym to build up my muscles (NOT!!) I have to say I was pleased with my decision and when I did a few test shots and viewed them on my Macbook Pro's Retina screen an involuntary WOW! left my lips. Picking my jaw up from the floor I realised I was in for a treat with this lens.
Why is it so good? Well it's super sharp across the frame including wide open, but has a velvety smoothness at the same time. Difficult to explain, but the files it creates are things of beauty. The colours are again just beautiful. Leica colour is legendary, punchy without being gaudy and I'm convinced I can see more shades than with any other camera / lens combination. Construction is seriously heavy duty and as usual with Leica it's the details that count. The focusing and zoom rings are smooth, yet with that light resistance that makes them feel that they will last forever. Putting on the lens and locking it feels super firm without having to use any force.
When I first tried the AF I thought the lens wasn't working properly, but then realised that it was focusing so fast (and completely silently) that my brain took some catching up to see it hit focus. It is the fastest focusing lens I've ever used and while I rarely need that kind of speed, it's handy to have it.