What are some messaging apps that are compatible with //both// iOS and Windows?

My long distance friend and I would like to converse daily. We message on Skype, but it's very glitchy. She has a Windows tablet, and I have an iPhone. I also have an ANDROID tablet if you can think of messaging apps that are compatible with either iOS&Windows or Windows&ANDROID. Please give us some apps. I can't seem to find any.

Viber and Line can be downloaded on both mobile and PC

Facebook messenger?
Whatsapp?