For "normal" focal length, I really enjoy using the 50/2.8 Elmar-M V2; as far as I can tell it's relentlessly sharp and its collapsability makes it extremely compact on any M. On my M8, I use the CV 35/2.5 Color Skopar PII, with the crop factor provides the equivalent of 47mm.
For a wide, it's the CV 15/4.5 Super-Wide Heliar, hands down.
50mm lux asph for best combination of useful field of view, image quality and particularly beautiful bokeh. But for portability 35mm summicron v4 (v4 just happens to be what I have but any version would be great).
My current favourite is my 35mm Summicron. I think it's the third version (spring-loaded separate lens hood with vents). I haven't had it long but I love the fact that I can do anything from landscapes to street to portraits with it - without compromising image quality - and it's possible to isolate backgrounds when shooting wide open.