Watakushi: This would be if someone like the Prime Minister or a President was around. You would use this to show the ultimate respect.
Watashi: The most formal in most situations. Since being polite in most situations is part of Japanese culture, you would probably want to use Watashi.
Boku: This technically isn't rude, but its more informal for close friends and family. If you are at something low key like a sporting event or a cookout, then Boku would be more acceptable (though when in doubt, use Watashi). But if you are at a job interview, you would want to stick with Watashi.
Ore: This one is interpreted as boastful and crude. No etiquette.
So in summary...
Watakushi: Salutations my most esteemed guest! What pleasantries surround you today?
Watashi: Greetings sir or madam. How are you?
Boku: Hello. How is it going?
Ore: Yo! How it be?