If you don’t mind a learning curve and having to use the Web interface (because there’s no native mobile app): (streams). From Friendica’s creator.
If (streams) sounds good, but you need a shit-ton of extra features on top (and be it diaspora* connectivity), and you don’t mind an even steeper learning curve: Hubzilla. Also made by the guy who made Friendica.
If you absolutely, absolutely, absolutely must have a dedicated native app on your phone, you’re on Android, and you can live without features such as nomadic identity, multiple channels per account and advanced, fine-grained permission control: Friendica.
If you absolutely, absolutely, absolutely must have a dedicated native app on your phone, but you’re on iOS: Wait for Relatica to have a stable release, then Friendica. (Caveats see above.)
Forget diaspora*. It’s fading out. Shortly before New Year’s Eve, a bunch of big diaspora* pods shut down, and at least according to one stats site, diaspora* lost more than haf its users.
And Pleroma is a Twitter replacement that, just like Mastodon, started out as an alternative UI for GNU social.
Lemmy users = Redditors = geeks.
OP is talking about people for whom the Internet is Facebook, Google, YouTube and Amazon, and who have always only ever used phones and never in their lives laid their hands upon an actual computer.
People who had their Gmail account registered and configured by whoever sold them their phone (or one of their past phones).
Anything that goes beyond “load an app with the same name as the thing you want to use from the App Store, user name, password, go” is too complex for them.