1. Portugal. Peace-loving, tall player here. The peacetime GG's help me defend, keep a larger standing army, and shape a city's terrain the way I want. Science and Gold are also great yields for staying alive. Nau's are also really great because they pay for themselves, allowing you to have a huge standing navy scare the AI into being friends with you.
2. England. The early spy I almost always use for choosing CS allies. Can create a nice buffer zone against hostile civs, and taking Militaristic CS lets you get science and free units (so basically science and gold like Portugal). Then the maintenance reduction for naval units for, again. a large standing navy. After White Tower, you get tons of Science and Gold.
3. Byzantine. I really enjoyed a recent playthrough where I optimized Great Prophets. It was cool having a small country with like, 16~ Holy Sites and a disproportionate amount of votes, Faith, Culture, and Tourism. Kind of felt like a 'roided up Vatican or something!