Windows/Mac/Linux: Security is naturally a balance between convenience and safety. If you prefer to stay on the safe side of that spectrum, your chats can't get much more secure than Bitmessage.
While the interface is a bit intimidating to people who are used to things like Hangouts, the app allows you to send encrypted messages over a peer-to-peer network. There's no central server where your chats are stored, and it won't be easy to sniff those messages along the way. You can even set up a proxy for your messages to bounce through.
While it's not the simplest app in the world, it's great if you have a group of people you need to communicate with and absolutely can't risk having your messages read.