Yeah, that was definitely one of my favorite moments in superhero history. It's right up there with "one punch," imo.
Justice League Unlimited, Season 2, Episode 5 ("Flash and Substance") http://www.imdb.com/title/tt0618137/ http://en.wikipedia.org/wiki/List_of_Justice_League_episodes#Season_2:_2005.E2.80.932006
The irony in that scene is Mark Hamill voicing The Trickster - The role he also played in the live action Flash series. The art there even looked a bit like him.
Why is that irony (i.e. an outcome that contradicts one's expectations)? "Flash and Substance" was all about paying tribute to the Flash's history, and the live-action series was part of that history. And it was Hamill's role as the Trickster that led to his invaluable work as the Joker in the DCAU, so it's not surprising that they'd choose to pay tribute to it. Personally, I take the Hamill Trickster's appearance in that episode as proof that The Flash actually takes place in the DCAU. The live-action series has the same kind of retro-modern design sensibility as B:TAS, as well as using Shirley Walker as its composer, so it fits pretty neatly (although how Barry hands over the Flash mantle to Wally remains a mystery). True, there is a continuity glitch or two, like Barry walking his dog past signs for Superman and Batman movies, but there are other, similar glitches within accepted DCAU shows, like an early Static Shock episode (before it was folded into the DCAU) having its characters mention Clark Kent as Superman's secret identity and a Superman: TAS episode with Lois referencing Wonder Woman before the latter debuted in JL. So it's not a dealbreaker.
Below is my favorite Flash moment from JLU, but it takes some explaining. The plot of the episode is that the Flash and Lex Luthor switch bodies. Flash realizes that if he lets it be known that he's in Luthor's body, the villains will take him down, so he tries to hide out and pretend he's Luthor: [yt]http://www.youtube.com/watch?v=VaI1iBw0K64[/yt] The parts with Lex Luthor in Flash's body are neat, because Michael Rosenblum, who played Lex Luthor on Smallville, voiced Flash for Justice League and Justice League Unlimited.
Sorry, I wasn't as clear as I could have been there. I know JLU is part of the DCAU. What I meant was, I thought it was a DCAU series, I just wasn't sure which one.