Picking "the best" lenses for everyone is, of course, impossible. Lenses, after all, exist to serve different needs.
The "best" lenses regarding image quality are the Super High Grade lenses. However, due to their size, weight and price, these are not the options that would make sense for the overwhelming majority of users.
Best Normal Zoom
Olympus 12-60mm f/2.8-4 is the best 4/3 normal zoom. Reasonably priced, has great zoom range starting at the wide 12mm (24mm equiv.), sharp. There is very little bad to say about it.
- Olympus 14-54mm f/2.8-3.5 - a bit brighter at all focal lengths, cheaper, more ubiquitous. Less zoom range, you can really see the difference at the wide end.
- Panasonic 14-50mm f/2.8-3.5 - very similar lens to the Olympus 14-54mm, it is usually not reasonably priced.
- Olympus 14-42mm f/3.5-5.6 - while much darker at all focal lengths, it is pretty incredible how sharp this tiny lens is.
Olympus 50-200mm f/2.8-3.5 SWD is the best 4/3 telephoto zoom. Still reasonably priced, and of reasonable size/weight, it is a super sharp lens, that @200mm and f/3.5 is very bright when zoomed in.
- Olympus 50-200mm f/2.8-3.5 - the older version of this lens lacks the SWD autofocus motor and thus focus slower, but other than that, performs very similarily.
- Olympus 40-150mm f/4-5.6 - this plastic fanstic is very affordable and sharp. If you don't want to carry around a large telephoto, this is your lens.
- Sigma 50-500mm f/4-5.6 - if you are buying into 4/3 to make full use of the 2x crop factor and thus massively zoom in, this may be the option you are looking for. It is, though, much darker and sharpness is not on par.
- Olympus 70-300mm f/4-5.6 - this kit quality lens is cheap, light, and allows you to zoom in really far. Sometimes that's all you need. Sharpness, f-stop and autofocus speed is not on par with the 50-200mm SWD.
Best Wide Angle
The Olympus 7-14mm f/4 is the best 4/3 wide angle lens. Out of all the Super High Grade lenses, this was the cheapest and smallest one, and since, unlike the other lenses, it has been replaced with a m4/3 version, so you can have it much cheaper than any other SHG lens. Excellent sharpness corner to corner, the only bad thing to say is that it doesn't take filters.
- Olympus 9-18mm f/4-5.6 - not as sharp corner to corner, this lens is nevertheless much smaller, almost 3x lighter and can take filters.
Best Portrait Prime
Out of all the super fast aperture primes, only the Panasonic 25mm f/1.4 is fully usable wide open. The only downside to this lens is that it is pretty hard to find.
- Olympus 50mm f/2 - while the focal length is quite long for portraits and the f/2 aperture is nothing to write home about, this is still a fantastic lens, sharp and usable at f/2.0 and a 4/3 classic.
Best Super Zoom
The Panasonic 14-150mm f/3.4-5.6 is the best super zoom for 4/3, delivering a very useful zoom range and fantastic sharpness.