Boudicca: She converts barbarians to your army. A really fun ability, could be a cool mechanic for some civ too. Stamford Raffles: Absorbs a city-state into your empire. Powerful late game merchant, it's also nice to read and learn about the people and learn of the history behind them. I like that in Civ VI there are some surprising choices in generals and admirals, not just the usual Rommels but some unusual picks I never heard of.