Some ground rules. Post IMP, full upgrades. No hero units. Only standard units a civ can make in a normal ranked game. Other than that you can pick whatever civ and unit you want.
Replaced the champion with your chosen unit. Who would you pick on the small island or the big island to act as your champion in this 1v1 duel? On the big island you have room to move your unit and kite whatever you’re facing if you are using a ranged unit.
Edit: You are blue player. Red will randomly dice roll what his unit and civ he will be. But you can choose.
Small island: Elite War Elephant, probably.
Big Island: I don’t know. There is some rock-paper-scissors going on. I’d be tempted to pick the Elite Camel Archer. It probably gets beaten by the Elite Rattan Archer, though.
For the small Island an Elite War Elephant is easily the most powerful unit with over 600 HP it’ll kill any unit in the game. It’s hands down the most pop efficient in the game, but its production costs (cost of unit, time to train, cost of upgrades) makes it prohibitive to employ in most scenarios.
For the large Island I’d say the Elite War Wagon is the best unit. It’s a highly pop efficient unit with a lot of HP and pierce armor so it devastates any ranged unit out there and it’s fast enough to kite higher HP melee unit or anti-archer units whereas no faster unit in the game will be able to defeat it 1v1 or kite it well enough.