OK, lets say that you are playing with 5 other players. You've never played with them before and are unfamiliar with their playing style or strategic tendencies. By luck of the draw, YOU have been blessed (or cursed) with the FIRST CHOICE of territories. Nothing has yet been selected. What is your first choice and why?
My first choice is NORTH AFRICA. With North Africa you still have an option as to what you will try and secure first. Africa itself, being the obvious option with South America being equally doable. Both continents are pretty manageable in terms of fortification. I'm not saying you immediately go for both, just that it's nice to have the option to follow the path of least resisitance once all armies are placed.
One could make the argument that Australia is equally attractive, but what to do once Austrailia is yours? Going through Asia is likely your only choice if you do not have a stronghold elsewhere. North Africa gives you a shot at one of the more desirable continents in the early stages and still leaves you with some flexibility.
You could almost say the same about Brazil, but I believe taking SA from North Africa is a tad easier than going through Africa from SA.
On a side note: My son bought me a "Nostalgia Risk" board for Christmas. It has real wooden pieces and a replica of the original 1959 map board. He's only 4 and doesn't know about Risk at all, he recognized it from from me playing online and told his mom that's what he wanted to get for me so we could play together. I can't wait until he is old enough to understand it a little better. what a kid.