Recently I've started using two lenses, designed by Leica and manufactured by Panasonic, for the 4/3 standard. These are the Elmarit 14-50 F/2.8-3.5 and the Summilux 25mm F/1.4.
Both are impressive lenses and amongst the best I've ever used.
Panasonic are a late entry to the serious photography marketplace and it seems that they are more serious than most. With their cameras & particularly the lenses they develop both alone and with Leica, they seem to be concentrating on quality & innovation rather than mass market appeal.
I'm very impressed by the 14-140mm "kit lens" for the GH1 also.
This is encouraging and long may they continue to service this niche market. As someone who has at times endured the plastic non-entity kit zooms, produced in their 1000's by Nikon, Canon etc. this is a refreshing change. Its frustrating to realise that many photographers who buy DSLR's will never use anything else other than those ubiquitous 18-55's and 55-200's.
The pleasure at seeing an image taken with a really good lens is one of the reasons I continue to be obsessed with photography. Even if there are few of us who will see the difference, or more importantly care about the difference, to me its worth doing.
I was impressed by the kit lens on the G1 also, but the 3 I have mentioned are star performers.
Not light, not small & not cheap certainly but any lens that makes me want to get out and take pictures with it gets a high rating from me. All of these lenses fall into that catergory. I have even left the Leica M8 at home to go out & use them & thats high praise indeed!