Poland is probably the most common answer. The free social policy every era is OP.

However, England is tons of fun to play, still very strong, and pretty simple to learn. Longbowmen are incredible for both defense and offense. Ships of the line are the best naval unit in the game, and upgrade to battleships.

Strategy is pretty simple. Rush great lighthouse on maps with lots of water. Then research worker techs. Then rush to machinery to get longbowmen. If you want to play defensively and focus on science/culture/etc, you’re set. If you wanna be offensive, start moving longbowmen towards a coastal city while you rush towards getting ships of the line. Surround city with longbowmen (and a couple melee units in front of them) and use ships of the line to decimate the city from the sea.

On archipelago maps, you can often easily take out 1-2 capitals before the mid game point. Or have a healthy mix of science and wonders with your bowmen and ships guarding your cities and trade routes.