I found it is a bit odd that you couldn't pick your supervisor. I think in most universities who you want to work with are up to you and the supervisors who wish to accept you. The best advice for finding a good supervisor is to ask current graduate students. Some supervisors are well known to be good supervisors among grad students whereas others are known for being super mean to graduate students.. Current grad students usually know who are good at supervising. Also ask about the style of supervising. Some supervisors are very hand off but you get little support from them. Others can help you a lot but may impose their ideas onto you.