I'm thinking about creating an adventurer/dungeon crawling character. Which caves/ruins/dungeons are a must visit?

misread the thread*

Sorry I read that you were creating a dungeon/ruin etc.

What?

All of them.

Explore Blackreach at least once.

Any Dwemer ruins. They're the most diverse, with the Dwemeri architecture on the higher levels and the Falmer-ruled 'underworld' of natural rot and growth.

Mzinchaleft (spelled wrong, but you should be able to recognize it) is a good Dwemer ruin with a lovely sword at the end...