lens that is ! For me it would be my trusty 35mm 1.2 While I have several others of various focal lengths it's the 35 that is almost always on my camera. Granted , ones primary subjects being shot determine as much, mine is RR photography, it's just my go-to..... Back in the day my grandfather shot some amazing pictures with his trusty 'brick' (Argus C3 rangefinder), those of you old enough that can remember that camera.
I'm a fan of the 35mm. I call it my 'mighty 35'. Does good work. A lot depends on where I stand to where the subject is. Hence, my other lens.
when i did this Question several years ago i had the primes down to a choice between my modern 32mm ƒ1.4 canon efm and a vintage 50mm pentacon ƒ1.8, i think then the pentacon won on sheer smiles the zoom lenses came down to the small and compact efm 18-150mm and my daily sigma 100-400mm, most of my photography is wildlife so the 100-400mm had to win there but it was a tough choice today the 32mm would win over the 50mm vintage purely down to my eye sight and the more useful to me now modern AF so the question is which lens from the 32mm vs the 100-400mm as my only lens, it would have to be the 100-400mm because i cant get the reach with the 32mm but i can make a wide shot with the 100-400mm if i really wanted to with multiple images i'd miss the low light performance of the ƒ1.4