The easiest to setup should be port-forwarding on the router. If you trust putting your server on the internet, then that's the way to go. But it really depend

The easiest to setup should be port-forwarding on the router. If you trust putting your server on the internet, then that's the way to go. But it really depends on the use-case. HTTP from behind NAT - that's easy, just port-forward. If you're talking about SSH access, then you have a few more options that you might want to explore (port forward, or tunneling to an external host). If you're talking more than one host behind the NAT, then you have another set of possible solutions (reverse-proxy HTTP servers, SSH gateways, etc...). Care to give us more information?