I always try and find an Invade & Annex server with a fair amount of people in it.
There are lots of friendly players in these servers and you really form a great sense of teamwork.
A few times I've been teamed up with a guy, can't remember his screen name, who is a highly organised team leader and those were some of the best gaming sessions I've ever had.
Endless amounts of fun can be had in a fairly "serious" game, you just gotta look for the right server.
Wasteland servers are also loads of fun but can be hard to form up a team.
I'm keen to get into some Altis Life servers, but they seem pretty complex and haven't given them a go yet.