Whats the best way to answer an open question with bullet points or full sentences?

Personally I prefer full sentences. One of the skills recruiters are looking for is your ability to express yourself clearly. This is more difficult to do when you have a tight word limit, so is all the more impressive when done well. Im always keen to find out about the benefits that applicants have gained and the skills that they have developed from the experiences that they have had. Full sentences enable applicants to communicate this better than does a list.

That said, a list may be appropriate in response to some questions, perhaps when describing the responsibilities you held in a particular job, but generally full sentences enable you to sell yourself better to the recruiter.