I'm fascinated by South America. What are the Best sights to see and countries to visit?

I've only ever made it as far as Mexico and I'm very keen to check out South America, I'm just not sure where to go to see the best of it. Brazil has always seemed the place to go, but Machu Picchu in Peru seems amazing, Argentina beautiful and far further South of the equator than I've made it before, the list goes on... I can't decide where to choose, and doubt I can cram more than one or two countries into a holiday. They all look so fascinating. --Some details: I love travelling, don't mind roughing it a bit, don't fancy somewhere with a lot of trouble, like Colombia, I'm from England, I love wildlife and nature but I'm arachnaphobic! Thanks, in advance