Griffin the Invisible Man himself has many great moments, but by far the best would have to be how he takes revenge on Kemp; quite apart from not being fooled by the police's attempt to protect him, he waits until Kemp feels as though he's managed to get to safety, and then reveals that he's sitting in the backseat of the car and has been following him this whole time. Then he ties Kemp up, drives the car to the edge of a cliff and...
Griffin: I hope your car's insured, Kemp: I'm afraid there's going to a nasty accident in a minute... a very nasty accident.
Kemp: Griffin, I'll do anything, everything you ask!
Griffin: You will? That's fine. Just sit where you are; I'll get out and take the handbrake off and give you a little shove to help you on. You'll run gently down and through the railings, then you'll have a big thrill for a hundred yards or so 'til you hit a boulder, then you'll do a somesault and probably break your arms, then a grand finish-up with a broken neck! Well, goodbye, Kemp. I always said you were a dirty little coward- you're a dirty sneaking little rat as well! Goodbye!
(He takes the handbrake off, letting the car roll off the cliff- Kemp screaming all the way, dampened by the sound of Griffin's maniacal laughter.)
Hobbes is injected with a serum that turns people into geniuses but also turns them crazy and suicidal. He knows how to make a cure but he doesn't want to. So Darrien injects himself with the same serum, which is lethal because of the invisibility gland, convincing Hobbes to tell Claire how to make the antidote. At which point he screams like a baby and yells at the doctor "GET THIS CRAP OUT OF ME!!"
In an episode where Darien poses as a little girl's invisible best friend 'Ralph', a sniper is shooting at her while she's hiding behind some rocks. Darien is on the edge of quicksilver madness when he goes invisible and beats the living piss out of the sniper.
Sniper: What the hell are you?!
Darien becomes visible
Darien: I'm Ralph.
In penultimate episode, Big Bad #1, Arnaud, is having brain surgery courtesy of Big Bad #2, Stark. The surgeon explains his concerns when Stark points out that as long as the macguffin is safe, Arnaud is expendable. At which point Arnaud's tape recorder goes off saying "It's 11:00 so I assume you'll be getting my surgery underway. I'd like to draw your attention at this time to the C4 plastic explosives in your belt packs!" And he says it in such a happy, chipper way, too.
Stark is actually impressed by this, and it usually takes a lot to impress him.
In the "Pilot" movie, Darien snapping the neck of the mook that gunned Kevin down.
From "Ghost of a Chance," Darien successfully pulling off his Scooby-Doo Hoax, while exposing Alianora's.
"If she's a ghost, how come she's so afraid of this gun?"
In "Eye of the Beholder," Darien is blinded by Professional Killer, the Chameleon. (Quicksilver ensures it's only temporary and Darien can still see while invisible.) Later, as he tries to stop the Chameleon from carrying out his hit, Darien goes Quicksilver Mad and turns invisible before really cutting loose.
Chameleon: I can't see you! Darien: Yeah, sucks, doesn't it? (pow)