My vote for the best rule change to a professional sport: the overtime rules in the NHL.
For those who don’t know, when an NHL game ends up tied, they do two things different than they used to: 1. They have 4 people on the ice per team instead of 5 (less if there’s penalties). 2. They give both teams 1 point in the standings, and give an “extra credit” point to the team that scores in sudden-death overtime.
This means that there’s no downside to losing in overtime (you already got your point for a tie), so combined with less people on the ice, you get incredibly exciting, offense-oriented hockey.
This year’s crackdown on interference seems to have opened up the rest of the game, so kudos to the NHL officials!