Brief Thoughts on Some Ships:
USUK: They're best friends who are obsessed with each other and always looking for excuses to hang out. They tend to info dump all over the other. They're stubborn, kind of petty, very protective, and have a hard time admitting how much they desire the other's company, praise, and attention, but they're working on it because they care deeply about the other. They're in love, your honour.
PrUK: One of the best slept on England ships. Two wild guys that go out to loud punk rock concerts and have drinks together, then quietly recharge at home (e.g. writing in their journals, reading fairy tales, and sewing). Fun personality combination? Check. History together? Check. Canon interactions? Check. Though they really could still use MORE canon moments, given how much potential they have. Pirate x priest with these two is always excellent.
EngBela: The sexy Addams Family-esque couple who live in an elegant gothic mansion and pass time indulging in their hobbies, such as fortune telling, talking to ghosts, singing, and listening to rock music.
FrUK: Used to be an OTP, now I mainly like them as close platonic frenemies. Romantically, it's better in theory than in practice – that is to say, it has potential, but I'm not a fan of most depictions. Also, nyo FrUK is superior.
AmeBela: The dorky hero and goth girl. They confused and exhausted each other at first, but eventually developed an understanding and started getting along pretty well. He can fix her. She can make him worse. Alfred tries to encourage Natalya to form her own identity away from Ivan. Natalya encourages Alfred's suppressed yandere tendencies. She pegs him.
Romerica: They bond over sharing their cultures with each other. Alfred loves Lovino's food. Living in America provided all sorts of new ingredients for Lovino to experiment with in cooking, and he got to be good at something without Feli taking the attention away from him. Lovino is actually a fan of some of Alfred's home cooking as well. They both enjoy singing and dancing, and are fans of Dean Martin, Frank Sinatra, and Bobby Darin.
EngGer: They're both responsible and serious...until Arthur's wild side comes out and he starts teasing and playing games with Ludwig's head. Ludwig is socially awkward and a strict rule-follower, which Arthur can relate to, but he's better at being social and having fun than Ludwig, so he can help him loosen up – although in the bedroom, Arthur is more than happy with letting Ludwig be a strict rule follower. Ludwig is very obedient. Arthur trained him nicely.
AmeLiet: They're really domestic and get along well most of the time. They're comfortable enough that they can confide pretty easily in each other. Alfred is pretty protective of Tolys, particularly when it comes to Ivan. He doesn't like the way Ivan treated him and is quick to jump to his defence whenever possible. Additionally, Alfred is 'in love with the way Tolys undresses'. That's not questionable at all.