Jupiter 8. I do have some I-22 and I-50 (or is it I-10?) as well; they're sharp enough, but too slow. The J-8 is sharp and not bad wide open (adequate).
Oh, but its still hard to beat a sharp 50/3.5 collapsible on a Zorki C. That was one of my main shooters for a long time. It actually was pocketable (a jacket pocket).
I'd like to try a J-3 someday. Or its inspiration, the CZJ Sonnar. I think I'd use that extra speed.
I will get either an I-26 or I-61 eventually. I could ask here: which one is better? I kind of like the looks of the one with the really knurled focus ring (can't remember if that's the 26 or 61).