I am especially fond of the `get_disjoint_mut` APIs. They're such a neat demonstration that Rust's fundamental borrowing rules are not that bad; it's just that compilers aren't smart enough to realise when things are OK. Sometimes language/compiler improvements make things better (cf. the Polonius project in Rust), but sometimes just neat API design can save the day. (Still, this API wouldn't be possible in the early 1.x days of Rust; the language improvements elsewhere (i.e. const generics) made it possible.)
Edit: almost forgot to mention why this API is dear to me: 8 years back I was one of the first to suggest something like this: https://github.com/golddranks/multi_mut At the moment, it was not feasible for upstreaming to stdlib, though (in my mind, mainly because checking an arbitrary amount of disjoint borrows would've required heap allocations back then). Later language developments made it shine.