Just like the title says, what's your favorite bad-ass-mother-fucker scene? It doesn't even necessarily need to be a fight. A rousing speech, the completion of a long-laid plan - anything with someone acting bad ass.
My absolute favorite, and I still get all giddy everytime I watch this because it's The Major at her absolute best, is Kusanagi just going ape-shit on a mech suit. If you've ever seen Ghost in the Shell: Stand Alone Complex, you already knew damn well what I was talking about two sentences ago.
And speaking of Mechs and bad asses, no one does bad ass quite like Norris does in his Gouf Custom in my favorite Gundam series; 08th MS Team. Watch this guy single-handedly make everyone look like a bunch of fucking chumps.
That's right; cower! Be afraid! Die knowing you lost to someone who really knows how to use a Mobile Suit.