Amerika and Human Club are pretty big and fun. Human would be better if it weren't on the costanera. Getting a taxi back is a nightmare.
There's also Fiesta La Plop, which I believe is on a Friday night. It's at Federico Lacroze and something. La Plop is pretty cheap. $20sh pesos to enter, and drinks for about $10 pesos maybe? They check everyone's ID. There are lots of young, alternative guys and gals. I haven't been there since 2009. Just not my crowd.
My personal preference is Glam, at Cabrera and Aguero. It's not very big, but the crowd tends to be more upscale than the crowds at the other boliches. If you want to get in for free, send me a message. Drinks are 25 pesos, if I remember correctly.