It's very rewatchable partly because there are often five characters all doing something funny, even just an expression, and you can't catch them all the first time.
Thinking of this thread I remembered a bit of Dennis that really tickles me.
When he and Charlie are talking about Dee's new 'boyfriend' the stripper and how lucky he is that he 'gets' to take his clothes of in front of horny women, and Charlie derails it by talking about Dennis's rape by the school librarian. Dennis ends a rambling defence of that by saying
Dennis: "So yeah, I'll do it"
Charlie: "Do what?"
Dennis: "Strip"
Charlie: "Oh, you were talking about stripping? That's what this was about?"
Dennis: "Yeah yeah yeah yeah"
It's the way he delivers the final line, sort of frowning and biting his bottom lip. Can't find the clip anywhere, because why would anyone bother isolating that moment (Season 12 episode "PTSDee", if anyone wants to find it)? But I crack up every time. Love it to bits. So many little tics/quirks of phrasing from Glenn Howerton in that show really make me laugh.