I'm a Chinese and Arabic language major, took Japanese in middle and high school and speak conversational French. Japanese was by far the easiest to learn; besides Kanji, it's alphabetical with a simple grammar system. I still remember it and it's easy to figure out.

Chinese and Arabic are equally difficult but in different ways. Chinese isn't terribly hard to pick up but it gets a lot harder as you progress.

Arabic is difficult in the beginning but afterward you start to get a feel for things--it's a consistent system unlike English.