Hi Tim, I have three lenses for my 645 system, the 45/2.8 N, the 80/1.9 and the 150/3.5 N.
I had a 55mm lens previously, but for me it was kind of an in-between lens and I couldn't really get the feel of it. The 45mm works great for me for landscapes. The 80/1.9 I don't really use that much, but it is stupid-cheap so I bought it anyway. The 150 works great for portraits. I don't have he 150/4.0 so I can't really comment on it, but it isn't really that much less expensive the the 3.5.