There actually is a train to Cordoba, check out this previous post with a detailed description.
http://baexpats.org/...+train +cordoba
I think its very cheap, if that's how you define best option.
I've traveled with Betel bus company to BA, which is cama with wine, warm beef dinner and real silverware. It's also a religious bus company, so they do a group prayer as they leave the terminal. The dinner makes up for it though.