I don't have a lot of lenses, nor do I know enough to evaluate a lens technically. But favorite lens? That I can answer. Although my sig now shows a handful of K- and M-series primes, I haven't had these long enough for any of them to claim the place of favorite. In another few months I might have a new favorite. But for years my favorite lens has been the Sigma 70mm f/2.8 EX DG macro.
I bought my K10D -- my first and only DSLR
-- in 2007, with the kit lens plus the DA 50-200. A few months later I bought the Sigma. I didn't buy any more lenses till this year, so for a long time I only had three lenses to choose from. And most often that choice would be the Sigma, my highest quality lens and the one best suited to one of my favorite subjects -- wildflowers.
The lens is solidly built and looks and feels like a well-engineered tool for precision work. Not lens as art object, but a dependable working tool. It's fast enough to produce a bright image in the viewfinder, critical for macro focusing, and is a true 1:1 macro when needed. The working distance at 1:1 is quite small, so this isn't the best choice for insect macro, but for general close-up photography it handles well and is very versatile. I can't compare it to other macro lenses, but it has a reputation for top-notch sharpness even by macro standards and it has never disappointed me in that regard. Pleasing bokeh, also important for macro and close-ups.
70mm on APS-C is a generally useful focal length, so I also use the lens for other subjects, even landscapes.