A Leitz 40mm f/2 Summicron-C. The bargain among "modern" Leica lenses, it's a small fraction of the price of the equivalent 35mm*, offers impeccable performance and (from long experience with a Minoltina-S rangefinder) 40mm is a more useful focal length than 35mm, which distorts a little too much if you don't keep the camera back straight.
The gotcha is that only the forgotten Leica CL has 40mm framelines, and this is why the lens is cheap. The lens brings up 50mm framelines on other bodies and you basically need to shoot to the *outside* of the thick M3 framelines, plus a little bit. This isn't as big a problem as you might think, since the framelines are only an approximation due to the increased magnification at close distances. Remember that most consumer SLRs give you quite a lot more on the image than you see in the finder; some have only a 90% FOV. Or you can just buy a 40mm accessory viewfinder for the hot shoe, but that won't have parallax correction.
*currently selling s/h for more than the new price at some dealers, since Leica in Solms can apparently only crank out 63 lenses a week and demand (supposedly from Chinese nouveau-riches and people with a new digital body) has gone through the roof.