The episode with the drunken gunsmith. The end with Vash and the barkeeper.
Vash: AHAHAHAHA! HEY LADY WHATCHA WAITIN FOR, FILL ER UP!
(Barkeeper bonks him over the head with a beer pitcher)
Vash: I mean please fill er up miss, if you wouldn't mind.
Barkeeper: That's better.
Earlier in that same episode:
Vash (who's drunk off his gourd): Gimme...gimme back mah gun grandma...(gets hit in the face with his gun) * THWACK!*
And he still catches his gun!
Another episode, the famous "Genocide Song":
Vash: (singing) Total slaughter... total slaughter... I won't leave a single man alive... la dee da dee daa... genocide... la dee da dee daa... an ocean of blood... let's begin the killing time.
Thug 1: He's... singing?
Thug 2: What horrible lyrics!
Big Bad: Stay right where you are. He's just trying to scare you.
Vash: (sighs) I knew it wouldn't work.
And another episode:
Vash: THE DEADLY DODGEBALL HEAD! A simple technique to hold the ball in place with INTENSE SUCTION! Try this at home, kids!
Which brings in mind the first episode's Establishing Character Moment: the bad guy of the week is armed with a big-ass boomerang. Vash leaps over the weapon and sabotages the retraction mechanism so that it will wrap around the bad guy and squeeze him into submission.
Vash: (makes a cat-face) Dangerous toys are fun but you could get hurt!