Best way to listen to your own mp3s on your iPhone?

You can load them into iTunes if that’s your thing. I personally don’t like the UI, and have used foobar2000 for a good while (with a custom theme). You can load music onto it using iTunes, or from the files app on your phone. In foobar2000 you can also download from a upnp server, which is nice if you don’t have iTunes (like me). If you have Plex, the Plexamp mobile app works pretty well. It supports your playlists, downloading, and a pretty good radio.

Personally, I like to use Decoupled (https://decoupled.app/).

* Supported formats: FLAC, Ogg, Opus, Apple Lossless, MP3, AAC, WAV.

* Music sync through USB, via iTunes File Sharing or via FTP, WebDAV and a simple Web browser.

* Includes Dark mode.

I use the VLC app (I'm on Android). I use the SanDisk Ultra Dual Drive Go USB Type-C Flash Drive which has both a Type-A and Type-C connector so it's easy to transfer files.

I love Poweramp (i'm on android). Plex is second best but has stability problems.

iMazing

Been using it for years and years. Currently on my iPhone SE with a headphone jack. Total win.

iTunes made me insane. I had hundreds of albums carefully ripped during the before times. Plus a radio show archive.