Qualtrics is by far the best I've used as far as ease of functionality and customization with JavaScript and API. They also have a Users Group on LinkedIn, the users there are great in helping each other out where support lacks. Downside is its also one of the most expensive tools and they charge by response, so depending on how you're using it can get pricey fast.
I've also used Fluid Surveys (owned by survey monkey) and QuestionPro. Both are decent, and relatively cheap, but may be lacking in customization options.