If you really want only one outstanding lens, if you have the money I'd go with the Olympus Zuiko 14-35mm f/2.0.
Or if you like longer focal lengths the Olympus Zuiko 35-100mm f/2.0. I don't have any experience with these lenses but ive read alot of good things about 'em.
For a lower budget there always is the trusty 14-140 by panasonic.
There are of course other options like a J15x9.5B4 9.5mm-143mm f1.8(no full coverage in wider focal lengths).
[url=http://vimeo.com/20171062]B4 Lens on GH2[/url]
or a Canon C mount 17-102mm f2 (no full coverage around 85%).
[url=http://vimeo.com/28106895]Cosmopolitan x Charlie Yeung [GH2][/url]
but I don't know anything about these unusual lenses and I bet there are a lot more.
There still is the option to go with 2 lenses, even though this is against your principals.
I'd go with one zoom and one fast prime.
so you have one all purposes lens and one when you need the extra stops of light or the shallow dof (isnt that why we got into dslr's in the first place? ;p).
In one last note I'd suggest waiting for the X lenses before purchasing any lens.
