Mix from aggressive and defensive players. Reasons are a lot:
If you face defensive players, it is not hard to read most of the hands they play.
If you face aggressive players, you can manage to understand when they are
bluffing or not. Also, in many occasions, aggressive players when they loose a big part of their stack, in the next hand they usually are all-in, which means their hand is weak.
You must face every kind of players during the game, because that improves your play as well. If you play with only one type of players, the plays will be very simple,unattractive and understandable and the only way to get too many chips is both of you to have good hands.