In one JLA story (JLA #61), Batman gives the Flash what is supposed to be a reassuring hand on the shoulder prior to briefing Flash on his role in the plan. Flash's response: "You're touching me. Batman is touching me. I'm going to die, aren't I?"
"Not if you pay close attention."
As I recall, the plan involved Flash riding one of Batman's missiles into an enemy target. The look on his face was priceless.
Also from that issue, a great one-liner upon discovering a familiar villain in the midst of his villainy:
Wonder Woman: Abra Kadabra!
Green Lantern (singing): Gonna reach out and grab ya...
In JLA: Year One, J'onn transforms into Superman without anybody noticing and appears sitting at their conference table after the other members had been talking about asking Supes to join. After they stare at him in shock he then turns into ELMER FUDD. Especially funny because he has been quiet and relatively humorless up until that point.
J'onn J'onzz: You're vewwy vewwy qwiet. "Yoda! Do Yoda!"
Gail Simone's JLA comic The Hypothetical Woman has a scene where the League is planning their next move aboard the Watchtower...and while they're doing that, Wonder Woman surprises everyone with pomegranate cookies that she baked herself.
GL/Flash: Wonder Woman BAKED?!
And at the end of the scene:
Batman: Diana, Alfred will need that recipe.
Post-Flashpoint Justice League has the first meeting between Green Lantern and the Batman, having Hal ask what Batman's powers are before saying, "What, you're not just a guy in a costume, are you?" Batman responds by taking off Hal's ring.
And later, the Flash: "Batman doesn't have any powers? I thought you were a vampire or something."
This exchange in Justice League #4:
Wonder Woman: We need to get those men out of the helicopters and somewhere safe.
Green Lantern: I'll get them out of the choppers and to safety. Not to be a good guy like the Flash, but because I want to impress you. Most of what I do is about impressing people.
GL: No, I... what'd I just...?
WW: You were touching my lasso.
GL: So what?
WW: It makes you tell the truth.
GL:[looking off-panel] Are you laughing, Batman? At a time like this?
The cover of the in-universe book "Justice League: Gods Among Men" by the fictional David Graves. It features the Justice League fighting Starro... except for Flash, who's incapacitated in the monster with a starrosite on his face, while everyone else poses like badasses. It's almost as if whoever illustrated the cover forgot the Flash was a member of the League until halfway through drawing the layout, at which point he just squeezed him into the picture's remaining space.
Pretty much any of Green Arrow's attempts to join the League in issue #8.