I just took a trip to all three cities last month, and it is hard to pick from those three choices. However, given the hassle of getting a visa, Buenos Aires would probably be your best bet. It's a great city, especially on the weekends when its residents are very laid back, and you can wander around the different neighborhoods, visiting the markets that sprout up in plazas and parks. Also, the local cuisine is delicious, and the city is very, very inexpensive since the peso is so weak. Everything is an excellent value. I highly recommend this city.