Mandarin, hands down. although i know a few westerners who have mastered it, it's horribly difficult... even the chinese locals here (95% is chinese) have difficulty mastering the mother-language. their dialect has changed a little over the years and they are no longer considered fluent.
i speak 5 languages, but Manadarin is a killer.

__________________
Who are you going to believe, me or your own eyes? -- Groucho Marx