The Cavalier Fighter's Capstone feature Vigilant Defender seems extremely strong in combination with the feats Polearm Master and Sentinel. Vigilant Defender gives you an extra reaction each turn which you can only use to make opportunity attacks, polearm master gives you an opportunity attack whenever a creature enters your 10-foot reach, and Sentinel stops a creature from moving after you hit them with an opportunity attack. Put everything together, and as long as you keep hitting your OAs, no enemy without reach can ever hit you since they can't get within 5 feet of you. If you can find a way to increase your weapon's reach, you can even pull this off against foes that do have reach.