What's Your Favorite Online Survey Engine?

We're running a survey this fall of how scientists use computers, and while SurveyMonkey does most of what we want, it doesn't do everything. We would welcome pointers to other online survey engines that:
  1. Are very reliable.
  2. Provide a wide variety of tools (including "Rank the following in order of importance" with an option to mark some "not applicable").
  3. Don't fill 80% of the screen with banner ads.
Anyone have experiences to share?
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