@Samuel: The ones I mentioned as "the two best endings," probably.
If you're going for Good Life, kill no bosses and try to kiss the asses of the other families. You need to be bosom-buddies with at least two and preferably three, and you want to have Persuasion and Intimidation maxed.
If you're going for Boss of Bosses, there's only one way to get that, and it's to murder everyone in your way. Take every chance you get to murder a boss, max Firearms skill, and Fighting if you can.
The G-Man ending requires you to be an FBI Agent, which requires you to get a college education and flip early.
Not telling about the Senator ending. Make that happen yourself!