Was going to say churches/cathedrals, but why limit it eh?
Big fan of the big dom in Cologne personally. went there about two years ago, and let me tell you that is quite some church! just incredibly imposing and impressive, I was very taken with it. so much so that I now have a picture of it on one of our walls!
The Ismaili Centre in South Kensington is a really good example of a modern religious building IMO, it’s very sympathetic to it’s surroundings in a really heavy footfall location but also it’s very contemplative and has a really lovely roof garden which is somehow peaceful and quiet despite where it is.
Big fan of my local Cathedral in Rochester as the architecture is amazing, the space inside really impressive and they have hosted art events such as the Space Voyage Light show, The Museum of the Moon and even a crazy golf course.
Not religious, but I love religious buildings. Always amazes me that the UK is stuffed full of churches that are often about a thousand years old, and you can just go round and have a look at them for free. Provided they’ve remembered to unlock them that morning.
Greensted church in Essex claims to be the oldest wooden church in the world (it probably isn’t, but it’s still nice)
Don’t think I’ve ever met a cathedral I didn’t like, Il Duomo in Milan is particularly impressive - plus you can do a tour of the roof.
The inside is so much starker and weirder than you can tell on any photos, but I guess that’s good as it encourages people to just go and see it in person. Absolutely perfect place for a Le Guess Who style festival