that small question has a very complicated answer, some of which you may already know, but for a rentals strategy I would recommend trying to cover as many elements as possible, starting with the mid-range mana costs and low mana "filler" cards, then selectively choose higher cost cards based on preference (99pt mana battles exist, but are not common, so high cost cards are less frequently used)
additionally I would suggest a bias towards neutral cards, since they can fill in the gaps for things you missed.
this is all blind advice since the question is kinda vague, but maybe that helps a little.