Open Source Video Player
The thing that breaks a video player is rarely the common case - it's the one file with an unusual codec, mismatched subtitles, or odd container that the polished apps refuse, right when you need it to just play. The open source players here decode a far wider range of formats locally and let you control subtitle, audio, and rendering behavior directly, so playback depends on the software on your device rather than what a vendor decided to support.
Jellyfin
Free Software media server for managing and streaming your media from a dedicated server to apps
IINA
Modern media player for macOS with mpv playback, subtitles, playlists, and plugin support
mpv
Command line media player with broad format, codec, and subtitle support
Kodi
Free home theater media center for local, network, and internet audio and video
VLC
Libre media player and multimedia engine that plays most files, discs, streams, and devices
MPC-HC
Free and open-source Windows video and audio player with DirectShow filter support
Stremio
Addon-based media center for discovering, watching, and organizing video content
BigBlueButton
Open-source virtual classroom for audio, video, slides, chat, whiteboards, polling, and breakout rooms
Celluloid
GTK frontend for mpv that plays local files and URIs with mpv configuration and script support