50/3.5 collapsible Elmar LTM (featuring a bent infinity lock):
35/3.5 Elmar LTM (left) 35/3.5 Summaron LTM (right):

35/3.5 Elmar LTM (left) 35/3.5 Summaron LTM (right):

The Summaron 35f3.5 is OK. A bit flare-prone and with the hood, not exactly petit.
Another alternative is the VC 35f2.5 II - about as good as it gets in this size! Also the Canon 35f2 LTM mount. Less flare than the Summaron and a credible performer.
The miniscule Nikkor 35 LTM's are good, but they are getting so small that operating aperture rings, focus etc is more difficult - particularly on a M body.
Canon 35/2.8, all chrome, first version. 34mm filter ring. 15/16" extension from body. Tiny. Solid. Sharp. I have owned mine since the 70s.
I have the Canon 35/2, 35/2.8,and the small Summaron 35/3.5. The Canon lenses seem to be better performers than the Summaron has been doing for me.
The Canon 35/2.8 is tiny, sharp, solidly built,and affordable.
Another vote for the Canon 35/2.8, if compactness is important. The front element is recessed enough that you don't need a hood for it.
Ari -- You may be right. My Canon 35/2.8 (the earlier chrome one, as pictured above) wears a thin Walz filter all the time, and the filter ring makes the front element of the lens appear more recessed than it is. If I feel I need more flare protection, I've got a make-shift hood assembled out of step-up rings that works just fine for this lens (I also use it for my Canon 100/3.5). When all is said and done, though, this lens is very small, and very good.