what's the best rucksack for travelling in south america?

I'm planning a trip with my girlfriend to Latin America. We're going to be travelling by bus and trying to do it cheaply. We need to by rucksacks, so what do people recommend?

They have some made of carbon/kevlar that resist cutting and I suggest you hold onto it tightly...depending upon where and when you go, you could find that it takes a walk while you're distracted or gets removed from your person by that lovely guy on the other end of the Smith and Wesson model 10...keep a tight hold on your girlfriend as well and good luck!

I am a backpacker...I think the best ones shoul have a lot of pockets where you can distribute all your belongings easily... and they must be really resistant (think of all the times you will have to pack/unpack, load/unload them)...when they load luggage on the busses they don't really care, they just throw them in... have fun!