I agree with nearly every answer here. So, it looks like some Leica lens is "the best" for whatever application as long as it isn't something needing shorter than 28mm or so and longer than 135mm. Notice that nobody said the "best" lens is a 24mm or 21mm.... I think we know the short wide-angles are fine lenses, too, but they aren't our favorites because they are more rarified. To the extent the question means "what's your favorite that also has uncompromising quality?" my favorite is the 35mm ASPH, followed by the 28mm. The 35mm is just my idea of the most useful length all around; the 28 is at least as good a lens.
I have not used the 35mm Summilux. I bet that might be my favorite if I had, 'cause I like available light.
I have to say though, the 90mm Summicron is the one that kind of blows me away, delivering the occasional image that is like nothing else I have seen with a short telephoto lens. I have a couple of examples that I cannot post, because they are candids of other peoples' kids in town, but that's the kind of thing this lens excels at. If you are into candid portraits it will amaze you. (The extra speed helps with the "candid" part, 'cause you might be shooting in a darker place than if you had time to set things up.) People are raving about the new 75mm Summicron in the same way, I notice.
The most incredible, I think, is the Noctilux, because it is just so amazing to be able to shoot in such conditions. There's a minimum kind of condition in which you would use an F/1.4 lens with, let's say, ISO 400 or ISO 800 film. A whole stop darker than that is really dark, but you can still take pictures there, and they are beautifully lit... nice portraits with gorgeous out-of-focus backgrounds. Its MTF curves may look weird, but man, it is really a piece of work and does beautiful things. Many of us think of ultrahigh-speed lenses as being kind of a devil's bargain; this is not a lens of compromises but something of a marvel. I have used an Olympus f/1.2 lens quite a while, and I liked its ability to get me something in those same conditions, but the Noctilux is a whole other ball game. So I think this lens is the tour de force. I wonder if Cartier-Bresson ever tried one. Does anyone know?