Oberon, I am in zone 4a. The two clematis that do best for me are Niobe and H. F. Young. They are nearly indestructible. I have tried half a dozen others that did not work well. If you like the shrub or pillar type clematis, Alan Bloom and Mrs. Robert Brydon do well here also. I have clay soil, so I dig a pretty big hole and amend with compost. Other than that, I pretty much leave them alone except to cut back according to whatever pruning type they are.
I got a friend of mine a $5.00 H.F. Young at either Lowe's or Home Depot last year and it is gigantic....he also got Rosemoor at a fancy nursery, it was highly recommended, and it croaked. You never know.