I suggest to look at 50mm 1.8 if you're on a budget as it's sharper than 1.4. If you can afford more, look at 50mm 1.2 AI-S -this is what I have. If you want the best for low light and actually use 1.2 aperture and don't care too much about apertures above 2 then 55mm 1.2 is best.
I'm not sure there's any evidence for the 1.8 being sharper or cheaper--all I've seen to that effect is Ken Rockwell's opinion, which should be taken with not just a grain of salt but a whole box full of salt.
More important is that you recognize what the 50mm fov looks like on m4/3 sensors. This shouldn't be your primary normal lens. That's why Vitaly's request for clarification is important: recommended relative to what / for what purpose, etc?
I have the nikon 50mm ais 1.4. It's definitely a cheap and good lens on the GH2. However, at f1.4 wide open, like most lenses it doesn't perform too well. Lately, I much preferred to use the Nikon AIS 35mm f2 and the AIS 24mm f2.8 on my GH2. I like the focal length of the 35mm much better as compared to the 50mm on the GH2.
I just sold mine to get a new nifty fifty, 50mm 1.8g. Nikkor 1.4 Ai that I used to own, was a perfect lens when I use the 35mm adapter, letus and Nikon D3100. But, tried on my gh1, it was little soft at wide open. I tested with the 1.8d. 1.8d is much sharper at wide open on live view, but not sharp as Lumix 20mm 1.7. I think 1.8d or 1.8g is better choice if you don't really need 1.4. I don't mean that 1.4 ai is not a great lens... I might get a new 50mm1.8g after I get Samyang 35mm.
I'm on the fence with Nikon primes - I like the AIS lenses' manual focus build quality, but I prefer the aspheric glass in the AF lenses. Lately, I've been considering the Voigtlander Nokton 58mm f1.4 in Nikon mount:
@qwerty123 all the evidence you need is the link I provided:) Forget Rockwell Butt asked about the nikkor 50mm 1.4, not about FOV, but yeah you right about the 50mm not being normal, it's telephoto on M4/3
@stefanos thank you: >Wenn Sie das Beste für schwache Lichtverhältnisse und wollen eigentlich Benutzen Sie ein 1.2m Blende und nicht zu viel über Öffnungen oberhalb von 2 dann 55mm 1.2 ist am besten.<
@Butt sounds like a very sweet deal. When you get to play with the 55mm 1.2, could you report back on how you like it and how it compares to 50mm 1.4 in sharpness?. I've done and I'm doing lots of research on nikon 50mm primes (I've chosen nikkor 50mm 1.2) and I have quite a bit of info on various 50mm like even nikon NOCT 58mm 1.2 but haven't heard too much about 55mm 1.2.
@stonebat - Rich colors and lovely flares, but that's what concerns me. The impression I get is that the wide-angle Nikon AIS primes are more susceptible to flare than modern lenses, due to their lack of aspheric lens elements?