My biggest gamble has been with Clematis montana, which 'everyone' said would not flower here. It does, but only ever couple of years - after a mild winter. I love it tough so am willing to live with the non-flowering years. The last good flowering year was 2010. The top of the arbour as seen from the pantry window:
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In July 2010 we removed the big New Dawn rose that shared the arbour with it. We had to severely cut back the C. montana in order to remove the rose. The C. montana has regrown nicely since but only produced a few flowers last May. It's been a tough winter (cold and dry) this winter so I don't expect much from it this year.

The arbour, and swag chains that link to it, now has multiple clematises to replace the rose so, even if the C. montana doesn't flower in May, I expect the other clematises to produce a show in June/July (I grow Group 3 hard-prune clematises but do not hard-prune them so I don't accidentally damage the C. montana. The Group 3s flower just fine without the hard pruning.)
Late June view of the top of the arbour:
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I like the Texensis clematis, especially 'Duchess of Albany'. This year they have a new one 'Princess Kate'. Then there is the Huldine, Viticella combo. Shock em.

But any clematis is a winner, given time.

Here is a link that might be useful: http://www.perishablenews.com/index.php?article=0024422

Let's try that link again for that 'Princess Kate' clematis.

Here is a link that might be useful: https://www.google.ca/search?q=clematis+princess+kate+zoprika&rlz=1C1CHMO_en&aq=f&oq=clematis+&aqs=chrome.1.57j59j60l3j0.6121&sourceid=chrome&ie=UTF-8