Finals.
Yeah, a series will help minimize variance, but if they two teams are close, even a 7 game series might not be enough. I also don't think every season there is a true "best" team, so the winner of the series doesn't necessarily tell you that anyway. A one game finals is more exciting, and the determinism of how many games are played is better. I find it distinguished mentally-challenged that the NBA finals could have anywhere from 4-7 games. If you truly care about minimizing variance, just have no post season and let the winner be decided by regular season record.