Try building the best decks that you can using only commons and uncommon cards. Then add rares and legendary cards when you need to. This way the decks don't seem to overpowered. Also you'll really want to look at the color schemes of the decks that you play since different colors yield different play styles (ex. R/B is usually aggressive while U/B is typically evasive and spell oriented). Build each of the decks to the best of your ability, this way no one deck gets all the favoritism. When everyone is playing at their best and trying out new things, regardless of who wins, everyone is gonna have a good time. A well made deck will lead to much enjoyment, that's just m opinion on that one.