All of these have workarounds (obviously), but here are the unstable features I'm using in my most recent (<10kloc) project:
#![feature(bound_cloned)]
#![feature(const_float_classify)]
#![feature(const_fn)]
#![feature(const_fn_floating_point_arithmetic)]
#![feature(const_generics)]
#![feature(const_panic)]
#![feature(once_cell)]
#![feature(option_result_contains)]
#![feature(or_patterns)]
#![feature(str_split_once)]
or_patterns are pleasant syntax sugar.
bound_cloned, option_result_contains, str_split_once are minor convenience shortcuts for things you can do slightly more verbosely on stable.
The other half are all about const generics and const fns. You can emulate some of them on stable, but the cost far outweighs that of switching to nightly in my case.