Websockets. (socket.io etc.)
Then use long-polling and (I forget the other one) as a shiv/polyfill for older browsers - if you require. A great polyfill is these guys polyfill.io, absolutely awesome what they do. I've tried to help them with a couple things myself :).
You could also use a product such as pusher and deepstream as examples - I recommend deepstream.
Pusher is good for plug and play, but deepstream is better for full control. So depending on what you want :)