Until recently, the Sony E16. While I was away for work, I decided to try my hands on some old manual focus lenses.
At the moment, it's a pre-war Zeiss Sonnar 50/1.5 that is stuck to my NEX. The Jupiter lenses are, unfortunately, noticeably poorer than the original (and the Jupiter-12 won't fit).
For M42, I have a Mir-1 37/2.8 and a Zeiss (not Jena) Tessar 50/2.8. Both are very nice lenses, but won't focus at infinity with my adapter. I've ordered a different one, and hope for the best.
Next time I go to work (it's an exploration camp, and I work there for 4-5 weeks straight), i'll probably get a 35 ZM biogon or a Voigtlander Nokton (either 35 or 40), while I wait for the announced native Zeiss 24mm.