In the US Military, it is korean. You have to not only consider the language, but what conditions you are learning them under. The korean curriculum is rediculously difficult. you only have 63 weeks to be able to speak fluently and listen fluently and read fluently in topics from your family and every day necessities to politics, accident and crime reports, and economics.

Also, the grammar in korean is so bass-ackwards from english I've known every vocab word in a sentence and still not understood it because of the grammar differences. Chinese has different grammar from english, but in actuallity it is also a bit similar. Plus they have words for rediculous things like 2 girls that fail their college entrance exam so they jump off of a pier and drown for 1 word... gah i'm sick of this language :-/

The hardest are easily Korean Chinese and Arabic though, as previously mentioned.