For a foreigner who has not yet completely mastered Japanese (and that includes me), watashi is always safe in formal situations but a little "effeminate" in non-formal situations for men. "Boku" is the safest, particularly if you are on the "quiet, polite" side, when used among friends until you are about 60 or so. After that, older Japanese men sometimes say "washi" :) If you are the "gangster" type or a tough guy, you might get away with using "ore" but that is a pretty tough term to use naturally as a foreigner unless you have the right personality, and the rest of your speech pattern is also tough/macho/a little less than refined to make using "ore" come off as natural to a native Japanese speaker.

私はbokuです。　ハハ　 （*∩_∩*）′

Thanks!

I totally agree with Brian. Watashi (or more formally, watakushi) is always the safest to use among them, irrespective of age or sex. Boku is a little more casual and is usually used by men when speaking to younger or inferior people. I do not recommend non-Japanese people to use ore to refer to themselves until they become very familiar with Japanese language and culture.