I use a Rodenstock 100mm f5.6 Sironar-N on 6x9, and also a Schneider 100mm f5.6 Symmar-S (I use the Rodenstock more often only because mine is in a more accurate shutter than my Schneider, there is not much to choose between the two lenses optically.) These are both lenses designed for the format: they allow movements on 6x9 but won't (quite) cover 4x5. Both are very good.
An older choice is the Schneider 105mm Xenars in various speeds. These are good lenses too, also not covering 4x5. Less coverage than the above lenses (about 120mm vs. 140+mm) but still enough to allow some movement on 6x9 (diagonal, depending on your back, about 98mm).
Some people like the Kodak 101mm f4.5 Ektar, though my own example of this lens is nowhere near as good as my Sironar-N, YMMV.
If you want something that will do double duty, a 90mm f6.8 Angulon makes a pretty nice standard lens on 6x9 and a lightweight wide for 4x5, and won't cost outrageous money.