I don't know that there's an exact definition, but generally it refers to a pitcher's stuff, usually their breaking balls since those are the most visibly filthy. Filthy, nasty, dirty, whatever you want to call it; the pitcher still needs to learn how to pitch.
I don't know if I'd say Johan's the filthiest. His circle change is certainly filthy, but he combines great stuff with great command, which is why he's so successful. I don't want to shortchange Johan, because he's so badass, but I guess I'm a little reluctant to call a changeup filthy because it's really dependent on having a plus fastball, which Johan has.
Pedro had the filthiest stuff I've seen in person during his prime, but I'm not sure who I'd say has the filthiest stuff today. It doesn't necessarily have to be the best pitcher. Felix probably deserves consideration, Liriano before his arm exploded, Peavy has great stuff. Off the top of my head, others might include Zambrano, Harden, Burnett, Cabrera, Kazmir...there are lots; maybe a reliever deserves the crown. I think this has been discussed before.