Based on the fact that you speak Spanish and English, I would recommend either German or Italian. German is very, very similar to English, both in vocabulary and in Grammar, and I believe that Italian is pretty similar to Spanish.
That being said, a Chinese dialect would definitely not be easy to learn, but speaking Mandarin or another dialect would definitely be very useful in the business world.
It seems you still need to work on English.
Japanese seems good. I always watch those japanese gameshows and wonder what they say.
Japanese and Chinese then you would be like the coolest person evaaaaa!
Italian, it would probably have the most similarities in grammar, construction and roots as English.
Most speakable(?) in America would be: German
Best for the future: Mandarin Chinese
Coolest: Japanese. Japanese has easy pronunciation, but a hard writing system.
If your first language is English, the easiest language to learn from that list would be German. If your first language is Spanish, the easiest would be Portuguese/Italian.
Russian, Arabic, Chinese, Japanese and Russian would prove a real challenge for a native English/Spanish speaker, since their structure, alphabet, and pretty much everything else is different to European languages. Not saying you shouldn't learn them, I'm just noting that they would be hard if you're looking for an easy language.