I love the one from "Heart of Gold" where Kaylee wants Wash to tell her she's pretty.
Wash: If I were not a married man, I'd take you in a manly fashion.
Kaylee: Cause I'm pretty?
Wash: Cause you're pretty.
I liked the scenes where Wash and then Zoe are being interrogated by the Alliance after they rescue the guy from the abandoned ship - and their utterly different responses.
And in "Our Mrs. Reynolds" where Wash thoroughly managed to resist.