Sounds like you are butthurt or your gf is argentinian.
Nope, I actually think it's a toss between Colombia, Venezuela and Argentina - Venezuela girls being hot and sweet, Colombians being bossy (in a good way) and hot, and Argentinian being the hottest but the bitchiest of the lot.
I can visit one South American country, where do i go?
Depends how long is your visit and what you want to get out of it. If you want an experience totally opposite of Europe, go to Nicaragua or Peru or Bolivia. If you want it to be as close as Europe as possible, go to Argentina or Chile. If you want a short, no fuss trip - go to Costa Rica. But really if you want to see an amazing country that has it all - Colombia!