People you will end up passionately hating. Actually people you don't like now or people you wouldn't speak to in the street could be quite good flatmates: you'll respect each other. There are subtler considerations, and the fault-line lies in familiarity. This is hard.
Generally it isn't a good idea to share with people you are or might be sexually attracted to, people who have their friends sleeping in the living room a lot, or where you are uncomfortable with the level of use of alcohol or drugs. Its important that you are clear about what standards of cleanliness is expected in the property, and kind of what time people are in bed by or at least silent by, during the weekday nights, and what happens in the accomodation or property at the weekend.
Don't share with people you aspire to be: its not going to miraculously 'wash off' on you. Nor will you necessarily appeal to their friends. Conversely don't choose flatmates because they represent no competition for you and you think they might make good 'accessories'.