40mm Summicron. The lens has a fistload of stellar traits, and no bad habits. It's sharp, crunchy, has exceptionally yummy bokeh, and has a very natural, "no tricks" angle of view. It's furthermore tiny, lightweight, and endearingly oddball with its practical, bespoke collapsible rubber hood and its rarer-than-hen's-teeth framelines.
I have a 50mm v.2 Summilux that makes even more jaw-dropping negatives (the detail sharpness is less crunchy, smoother, more Zeiss Sonnar-like, and the pictures it makes look more grown-up than the Summicron's), but it doesn't perform consistently across its entire aperture range the way my 40 does. It's also so heavy that it's not my go-to choice as a walking-around lens. The 40 beats it with the sum of its parts.