The most useful are perhaps the one's I'm using for search [0] -- chat logs, web annotations, book highlights, reddit/hackernews saves, tweets, etc. It's literally my external memory, I'm not exaggerating.

Everything in Promnesia [1] is super useful, makes my internet lurking much ... efficient? This word has somewhat bad rap, but it is what it is -- doesn't mean I don't have fun, just makes it easier to keep track and discover new things.

Not sure about surprising data sources -- I'm trying to integrate them all as far as possible and make sure they work for me passively, so I don't have to think about specifics :)

[0] https://beepb00p.xyz/pkm-search.html

[1] https://beepb00p.xyz/promnesia.html