You guys, you guys, regarding hard bosses, there is no need to go further than IWTBTG. For those of you that don't know what IT, the grand championship of hard mudder-effing games, is, look up "I Want To Be The Guy". Just google it, download the game, try it, hate me for introducing you to it. Personaly I can't get past the first screen, let alone get to the final boss. suprisingly addictive, of course, like all games of its kind. (Be warned: even short exposure may cause auto cannibalistic frustruation.)
Easy boss: Last guy of Condemned 2. And probably the Hivemind in Dead Space, once you get the hang of it.