3rd Season episode, "Last Kiss of Summer": Frank punching his brother Joe out, after nearly being killed by a shark. Up to this point, Joe has spent the episode trampling over the Justice Department's undercover operation and isolating himself from his family for his Roaring Rampage of Revenge. Frank's punch, followed by a "The Reason You Suck" Speech that's a spur-of-the-moment improvisation that still fits into the scam Joe's pulling on the Big Bad, is an awesome showcase of Frank's ability to think on his feet under extreme circumstances.
Frank: Let's not play games with each other. I've been on your case for 18 months and I've been treating you with kid gloves. Well, the gloves just came off. From now on you're not going to be able to change your socks without my knowing about it. If either of you try anything like that again, you'll find yourselves lying facedown in the sand. Got it?