Which out of the below listed South American Countries are a must visit and why???
Argentina, Brazil, Chile, Ecuador and Peru

They all are because they are all unique!

what if i have to pick 2/3???...can't do all due to the time limitations...any suggestions??

Out of the 4 coutries you mention, I have not visited the first 4 yet (going to the first 3 in October) However, we visited Peru and, if only for Machu Picchu, it is a MUST!
On that trip we also visited Lake Titicaca and the Uros Islands dsy trip was an unforgettable experience.

They are all must visit for different reasons

Really cannot pick one really should be based

on YOUR interests.

www.latinamericaforless.com

www.gotmyfare.com

both offer multi-country tours

For me www.visitperu.com for the Archeology/Culture.

IMO, Argentina is the best value for what you receive.

I would determine what you really want to see and do.

Argentina - Buenos Aires, Iguazu, Glaciers
Peru - Machu Picchu
Brazil - Amazon, Rio
Chile - Glaciers, Desert, Volcanoes
Ecuador - Galapogos

What stands out to you. All of these are major sites and must-sees on any list.

Go for Argentina!! So varied, includes virtually all landscapes and activities!

I vote Peru for the archaeology, colonial architecture, scenery, food and
Chile for the scenery, glaciers, hiking, wine, and seafood

which has beaches, culture, food, night life, and adventure (hike, kayak, white water raft etc)?

Lakes District/Bariloche has all of the above, except the beaches (lakes are warm enough to swim in Jan and Feb)

So many wonderful places.Ecuador for Galapagos, is the top of my list, and Machu Picchu, second. However for loving the whole country my favorite is Chile, maybe because I live in California and Chile has the same type climate (reversed seasons obviously). Drove from Santiago to Puerto Montt, a really wonderful experience.

I have only been to Peru and Chile. I LOVED Chile for many things (Glaciers, Valraraizo, Trip to the Andes, wineries, history, food, crafts) and it is more like a European trip than going to Peru. More stable Government, better economy, and you can rent a car and drive etc. Peru is more exotic and more archeologically important than Chile and has MANY VERY interesting sites, excellenet food and quite a bargain but driving on your own is NOT recommended and you need to adapt more to the countries peculiarities, including unexpected "farmers strikes" that can jeopardize your itinerary. SO, it depends on what is your particular interest and how adventurus you are. I want to send my -now- teenage boys to PERU when they are both in College because I consider it a far better life experience than many other countries.

I see nobody so far has voted for Brazil.. ..

Brazil has some rather annoying visa requirements.

For Amazon jungle excursions, you can also do this in Peru as an addon from Cuzco.

Its right that if you're American you'll need a visa for Brazil, but it has got a LOT to offer!

I would skip Ecuador (other than the Galapagos)...Peru was much more scenic and interesting,,,

Well, I think Quito is a wonderful city with beautiful old churches. Barefootbeach spent too much time in Quayaquil and that should be avoided, I agree. Mitad del Mundo is touristy but still kind of cool, and Octavalo has the best outdoor market I have seen anywhere.

I know this may sound really crazy to a lot of people...but its no harm to ask...has anyone done a combined tour of Brazil, Peru, Chile and Argentina in about 16-18 days?? Just visitng the highlights in each of these countries??

I would include Argentina Iguazu - NW in the way to
Bolivia... the lakes and altiplato and UYUNI...
LA PAZ... Peru - Machu Pichu...
Flintstones