Easy to use. Open source. Ad-free.
Don't select Video [best quality] unless you know that your video player can play MKV and WebM files (and Opus audio).
Don't select Audio [best] unless you know that you're able to play Opus (.opus) files as that's usually the format of Audio [best].
If using an iOS device, select Video [MP4] or Audio [MP3] as iOS does not support all media formats natively.