I like Boss Stress Dude the most. Him being built atop a historical background just makes him feel so much realer than the others. I feel like I could actually talk to him and he'd be able to justify why he believes what he does without implicating some relatively trivial personal issue. It's not just his ideology either.
It's also the methods he's taken to achieve his goals. Contrast him against Stain, who's actions amount to what's practically an awareness event due to him not actually producing the change he wants to see, and you can see what I mean.