I'd say "Message in a Bottle" counts. There are more than enough laugh-out-loud moments to qualify it as a humorous episode. Apparently, Robert Picardo was trying to cram as many jokes into it as he could. The part where the Doc tells the Mark II to stop breathing down his neck, and the Mark II says his breathing is a simulation, and the Doc says, "So is my neck, stop it anyway!" was written by Picardo. So many great moments in that episode. Seven remote-tasing a Hirogen, the Mark II trying to crawl into a Jefferies tube backward... I think I'm going to have to watch it again. "Our Man Bashir" is probably my favorite humorous episode, though. Kira's fake Russian accent and the hilarious bickering between Bashir and Garak never fail to crack me up.
the magnificent ferengi had my laughing harder than any other trek episode, i had to pause during that holodeck scene when they practice rescuing her. i was a bit young when it first aired though..
"The Royale" "In the Cards" "The Magnificent Ferengi" "Take Me Out tot the Holosuite" "Tinker, Tenor, Doctor, Spy"
I forgot about "Tinker, Tenor, Doctor, Spy"! Never mind, THAT'S my favorite humorous episode. That is pure, almost non-stop hilarity. I was crying with laughter the first time I saw it.
During the scene where The Doctor is telling B'Elanna that he can't be with her because Tom needs her and they cut to Tom sitting in the corner I lose it every time. There is just something about the look on his face and the little wave.
TOS: The Trouble with Tribbles, Spock's Brain TNG: Rascals, Sub Rosa (it's hilarious if you turn the right parts of your brain off) DS9: In the Cards (I thought they did a great job having a comedy in the foreground and the very serious buildup to war in the background.) VOY: Author, Author And of course every Mirror Universe episode