You typically want a mix of UDP and TCP (or sometimes a weird TCP-like monstrosity rebuilt on top of UDP). Taking great care to design your data streams to be

You typically want a mix of UDP and TCP (or sometimes a weird TCP-like monstrosity rebuilt on top of UDP). Taking great care to design your data streams to be self-correcting (e.g. transfer a world location where characters are going towards instead of which direction they are heading) can go a long way towards saving a ton of synching headaches, and enables very efficient networking. One-off events go over the TCP-like channel, but constant streaming data that naturally self-correct over time, like the example above, can benefit from being on the UDP channel.