My first choice would be a 150mm Sironar or Symmar. Like Shawn I have a 90mm, 210mm, and 300mm, also a 65mm and a 135mm

In terms of use I'd guess 80% of my my images are made with a 150mm, 17% with the 90mm and the other 3% mostly the 65mm and a few with the 210mm. However I'm beginning to use the 210mm nmore.

If I had to use one lens it would be my 135mm which I use on a second 5x4 camera (usually in Turkey).