The format depends on what you’re looking to do with your masters.
MP3s are great for sharing and to get an idea of how a work-in-progress sounds.
WAVs are the industry standard when it comes to releasing your music (be it digitally on platforms like Spotify/Apple Music or for physical items).
HD WAVs are needed for higher-end applications, like inclusion in a video game, movie or television broadcast, or for archival purposes.