Ask HN: Best chat solution for a creative/tech team on OSX?

We are a team of about 18 and have historically used iChat for our in-office chatting needs. Unfortunately, Messages has become incredibly unstable and multiple times a day, people message without the messages being transmitted. It kind of defeats the purpose of using an Instant Message client because you don't know if the person is just busy or if they didn't receive your message. Our biggest needs are sending screenshots, Audio/video chatting, messaging. Occasionally we need screen-sharing. Any suggestions?