Well they're all good, but I would suggest Japanese and/ or Korean, I'm learning both, but Japanese is better to do first because it will make Korean easier. I wouldn't like Mandarin or Cantonese or any other Chinese language because it take so long to learn, that you would almost never use it! Seriously, there was a guy that learned it for 15 years, and he's still not "fluent" only advanced. Besides, with Chinese, they have a lot of local dialects within dialects o.O Here are some good resources for each language:
Japanese Resources:
http://123japanese.com/
http://www.maggiesensei.com/
http://www.japanesepod101.com/
http://www.amazon.com/gp/product/0071747869/ref=s9_simh_gw_p14_d0_g14_i2?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=0XE0QPAPRGTATK4MDWET&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846
Korean Resources:
http://www.amazon.com/Survival-Korean-study-written-Arirang/dp/B001RFOD96/ref=sr_1_1?s=books&ie=UTF8&qid=1340319654&sr=1-1&keywords=survival+korean+stephen+revere
http://www.talktomeinkorean.com/
http://www.learnkoreanlanguage.com/
http://www.koreanclass101.com/
http://www.amazon.com/gp/product/0071737588/ref=s9_simh_gw_p14_d0_g14_i2?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=10X8QWXYXY1Y6SBKE4YN&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846
and buy random dictionaries to learn general words for each language, i suggest Random House or Berlitz.