Oh G-d. All of them, ever? I guess if I had to narrow it down, I would say any of the "holodeck something or other goes haywire, thus trapping the crew in a period costume drama from the Wild West/Victorian Era/Whatver." I mean, you think they would either shut the fucking holodeck down at some point, or prohibit personal use, or SOMETHING.
It's like they just stopped caring after a certain point: "Oh, shit, man, another season? Well, we can do at least 5 shitty Holodeck episodes, so that's 5 down; we'll figure out the plot later. Oh, yeah, and something poignant regarding Data's humanity is good for 2, even 3 if we make it a 2-parter."
And for me the worst episode was the naked one. Firstly, yeah, android subject to hallucinogenic water even though he's like, you know, an ANDROID? Also, what really got me was that they said they couldn't find any evidence in the Starfleet records of a similar incident--BECAUSE IT WOULD TAKE TOO LONG TO SEARCH THE RECORDS. Dude, we were able to run searches like that when the episode came out via GopherFTP for chrissakes.
The only thing worse than the worst TNG episode was Voyager, ALL OF IT. Oh did I fucking hate Voyager.
Luckilly, they redeemed themselves with DS9.