My vote is go for Europe and Asia. If you're aggresive and you attack constantly, Europe is perfect. It connects with all but two continents, so it's a great jump off point for attacks in any direction. Be careful though, because there are a lot of ways into Europe, so defending it is a headache. Asia works too, I guess, but it's even more aggravating to defend, and there's a lot more territories to fight over. Plus, you could build up a fall back point in Australia, and if things go seriously wrong, you could fortify Australia like there's no tommarrow and get ready to burst back onto the scene.
NOTE: I have no experience with diplomacy whatsoever, so keep that in mind if you're going to follow my advice.