Brawl of the Odd

Highly recommended. I've really enjoyed using this in my 5e campaigns; it's a great level of complexity for a mini-game, not too simple and not too complicated, and especially helpful to slot into a rowdy tavern in a pinch. For reference, these are the adaptations I made to fit in with 5e:

• Use ability scores rather than saves (which can be quite different numbers in 5e).
• Generate an AC for each character based on the standard unarmoured defence: 10 + Dexterity modifier.
• A Strike is implemented as a standard unarmed strike: d20 + Strength modifier + proficiency bonus, against the AC of the opponent. This means that high-Strength and low-Dexterity characters are more likely to successfully Strike and more likely to be struck; and the inverse is true for low-Strength and high-Dexterity characters.
• The Dexterity save made to Dodge a successful Strike is a contested roll against the opponent's Strike. If the Dexterity save is equal to the Strike, then 1 HP of damage is taken as per a standard Strike.
• I think it's best to proportionally scale HP values, centered on ~5. I wouldn't worry too much about the detail, but, for example, a 30-HP character vs a 35-HP character might translate to 5 and 6 HP in Brawl of the Odd, respectively.