Where's the best place in South America for an American to travel too?

A place where they are likely to speak english, or a good currency exchange, or American friendly.

Uruguay for sure....
If you are coming in summer, you'll find the most wonderful beaches in Uruguay. Punta del Este is the plus. Argentinians, paraguayans, and brasilians come to Uruguay to spend the summer here.
Good exchange for US dollars , friendly pple . and very safe and calm country.
Have many other options.Touristic ranchs, with typical countryside life....Places like "Mercado del Puerto" in Montevideo, to enjoy the national uruguayan food.."asado" (BBQ) and also tango
Enjoy !!!

Argentina fits your stipulations and is amazing. Rio is just a blast, even though they don't have as many english speakers....that doesn't mean they won't talk your ear off in Portugese. Oh and your money will exchange like 3/1 in both places.

Argentina!
The currency exchange is very favorable (US$ 1 = $3 pesos argentinos)
There are lots of tourist spots (Buenos Aires, Cataratas del Iguazu, Glaciar Perito Moreno, Mendoza, etc., etc.), english is spoken in all of them.
Things you cant miss out on: asado (typical argentinian bbq), tango.

Chile could be a good option too, as Argentina, tourist spots are very common in Santiago and also in near places as Valparaiso.

Argentina.
People are friendly and educated. the culture of the country is very european, very different from the rest of Latin America. Great pasta, pizzas and ice cream, and off course, beef!
the country is safe, affordable with many cultural, sports and touristic options.
you can go from Rain Forest and deserts, to lakes, rivers, ski resorts, glaciers in one country!
And Buenos Aires is one of my favorite cities in the world. People are beautiful, excellent restaurants, shops and hotels. Transportation is great, bus, taxi, subway or just walk. Incredible.
You should give it a try.

No need for me to elaborate,you have four very good answers.Having traveled extensively in South America the countries mentioned here by the other posters are exactly the same ones I would have recommended.I might add Uruguay to the list as another option.Why not take some time to visit each of them to determine which one is the best for "you" as they each have subtle differences.This is a very personal decision and I believe you will know instinctively which one is right for you after you visit them....Do your due diligence and then trust your instinct!

Chile, you can practice extrem sport as snowboard, sky etc. we have beautiful places, valle nevado, la parva, desert, cascades, la patagonia, nightlife, very similar to usa

PERU its the point :)
miraflores, cusco, mancora , el manu :)
and there many people speak english too :)

check it http://www.peru.info/peru.asp

goodluck