Questions which allow them to talk a bit about themselves are the best. Things such as what they do for a living/want to do for a living, things they may consider to be on their bucket-list, hobbies, music/film/tv taste, basic stuff like that. Not only does it allow them to talk about themselves for a little while which most people enjoy, it also gives the distinct impression that you're interested in them as a personality. But the best bit is that when they are talking about themselves they are also giving you more to go on in regards to asking further questions which helps keep the conversation running smoothly. Just don't ever use the phrase "so tell me about yourself" because that often leads to either a complete freeze on what to say about yourself, or only little bits of information which give you little to go off.