Mpv – A free, open-source, and cross-platform media player

It sounds stupid but the killer feature for me is possibility to have multiple subtitles visible, all easily configurable with a few keybinds (track, offset, position, size etc). No streaming service provides this, and they actively omit subtitle languages that aren't "relevant" to your geolocation. I cannot respect a service like that.

+1. This is a fantastic feature. I haven't even bothered learning the keybindings (perhaps I should), I just start with --sid1= and --sid2= and it works well enough.

Neither me nor my significant other are native English speakers, but we have different native languages. I'm comfortable enough with English, but like having English subtitles since I have a hard time with some dialects and occasionally just miss a word or two due to noisy audio. My SO likes having subtitles in their native language.

Being able to have multiple subtitles makes it possible for both of us to get the experience we like, at the cost of a little screen real estate. Well worth it.