What is your favorite camera lens and why/what do you use it for?

If I had to pick one it would be the 85mm f1.8. I use it for portraits & street photography.

Canon 85 1.8. Beautiful portrait lens on a crop sensor camera.

If one limits oneself to one photographic genre and style, it's reasonable to think that one would have one favorite lens. I have a broad repertoire. So I would have to say that the three lenses that I have are each my favorite depending on the particular subject and setting. Consequently, I can't answer your question.

My favorite lens is my fully manual 50mm f/1.4. Since it's too slow to use it for my line of work, the next best thing is my 18-55mm kit lens which does everything I need it to.

My favourite lens is an ultra wide angle zoom - a 10-24mm. I use it for landscapes, and landscapes are kind of my thing.

My favourite in my Zenzanon PS 150mm f/4, that I use on my Bronica SQ-A medium format film camera. I use it for street, candid,portraits, etc.

the one I have chosen to get the photo I want at any given time

My 18-200mm is used for everything from macro to landscapes.