Google Messages, the reason is that if another person has it, the messages will be upgraded to RCS and E2EE. It is now the default on Samsung devices. The spam filter works pretty well at killing random spam messages too.
No other open source messenger has any kind of E2EE and most of them are unmaintained eg qksms (Feb 2021), silence (Sep 2019) etc.
SMSes can be backed up easily on device with sms-ie, which also supports MMS message components unlike QKSMS or Silence.