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100/2, 90/2, 85/2 is my order of preference.
100 because it has ED elements, however it is possibly "too" sharp for portraiture work. The 90/2 is great, but more expensive than the 100/2. The 85/2 is a great lens with the focal length being perhaps more useful than the slightly longer lenses.
I've currently got a 100/2.8 silvernose (SC). Great for portraits, but perhaps a bit too long for my liking (for portraits). I think it's about on par resolution wise with the 85/2.
The 100/2 and 90/2 are very very good lenses in the Zuiko stable. They both cost quite high. And both are quite hefty in size. I have the 90/2 and the 85/2. So if $ is of no concern , go for the harder to get 100/2.
The 90/2 is a macro lens.
The 100/2 is incredibly sharp - a common 'complaint' amongst its users. Too sharp for portraits sometimes.
I have the 85/2 which is one of my favourites. Small and easy to carry around, it therfore gets to be used more often than the heavier lenses. Its not much bigger than a standard 50mm lens - thats the beauty of Zuiko lenses. I love it very much becos of its rendition of the out of focus areas - 'bokeh' as it is called. Very nice and smooth.
Zuiko f2 lenses are all worth buying and keeping. If U have been searching reviews and all that over the net, U would have known by now.
I also have a Zuiko 100/2.8 (boxed) that I am willing to sell.