The ~*ultimate*~ cross-platform messaging app is WhatsApp.
If you – and most of your contacts – have iPhones, it's a no brainer: use iMessage.
If you prefer features over security, plus texting, audio, *and* video chat, here are some options.
If you need end-to-end encryption, plus texting, audio, and video chat, here are some options.
If you just need a simple messenger for texts, here's what you should consider.
If you need a simple messenger for texts and end-to-end encryption, here are some secure apps.