Anecdotally I would say Dropbox. My friend who had a 4.0 in College who got offers everywhere else didn’t pass Dropbox’s interview. He’s a PhD student at MIT now.
Famously (until a few years ago at least), even places like MemSQL, Palantir, Quora, Yandex were all famous for interview processes tougher than the regular FAANG.
Rejection doesn’t mean difficulty. All of my worst students work for one of those companies. All of my best were rejected by the same company. Trust me; the competence gap was astronomical.
I agree with this, and I had students like that too, but no way, not in the case of this particular company. It had to be deliberate, or maybe they were just hiring their drinking buddies. I don't know, but it could not have been anything approximating engineering competence.
What is and isn't difficult depends chiefly on the candidate. Ask me about WASM and I'll fail miserably, but if you ask me about GPU architecture I may fare better.