Ok, I'm game for that.
S1: Progress
I have sat through this one several times, and I have to say, it was nothing but mind-numbingly boring every time.
I know it's often regarded as a fairly good episode, so I have watched it many times to make sure I wasn't just missing something. I still find it boring. Mullibok came across as a stubborn old man who just wouldn't move, and what Kira was doing should have had some impact, but it didn't for me. It really fits the damning stereotype of 'Bajoran stories are boring,' which I don't find in hardly any of the others. The only redeeming factor I find is the B-Story, which actually kept going when watching it.
Other honourable mentions go to (in order of season appearance):
Past Prologue - Fairly weak, nothing really doing aside from Garak.
A Man Alone - A generic plot, and a generic villain. This one was fairly boring too.
Q-Less - Could have been so much more. As much as I really like Q episodes, this one didn't work, Vash was annoying, and there could have been a better plot.
Move Along Home - Allamarain!
S2: Sanctuary
Season 2 was an ok season, but the bits in the middle gave us several sub-par episodes. The worst of these is Sanctuary. Translator scene was pointless garbage designed to give the episode more running time. The aliens were a) annoying as hell with their voices b) had makeup that was horrible to look at. The Promised Land thing came out of nowhere, and wasn't done very well.
Honourable mentions:
Rules of Acquisition - Aside from the Dominion mention, there is nothing redeeming this episode for me. Another horrible Ferengi episode.
Second Sight- Totally forgettable, aside from 'Let there be light', although not all that bad.
S3: Meridian
In one of DS9's best seasons, we have DS9's worst episode, in my opinion. Surrounded by great episodes, it takes the first and best prize for DS9's one bad episode a season. The plot is stupid, Jadzia's motives completely out of left field, she has absolutely no chemistry with her 'lover', and even Jeffrey Combs' B-Story fails to lift this episode out of the horrible depths that the A-Story takes it to.
The only other episode this season that even comes slightly close is Heart of Stone, and that's because of it reaching new levels of Contrived Plot.
S4: The Sword of Kahless
Hey, I, er.. liked The Muse, a bit. Season 4 doesn't really have any outstandingly bad episodes, but this one was boring, and Klingon. I think that says enough.
S5: Ferengi Love Songs
Again, I don't hate Let He... Ferengi Love Songs was terrible. Not really any plot, just the Nagus and Moogie getting together

(One of the many reasons I'm glad we now have a vomit smiley).
Honourable mention goes to Soldiers of the Empire for also being boring and Klingon.
S6: Profit and Lace

I think that expresses my feelings on this episode perfectly.
This season's boring Klingon episode, Sons and Daughters, gets the honourable mention.
Oh, btw, I liked Valiant.
S7: Prodigal Daughter
You'll have noticed I tend to condemn episodes for being boring, rather than having a stupid plot (although it's often a bit of both). In that case, Prodigal Daughter easily wins this season. A half-hearted follow-up to a good episode was trumped by Ezri's family issues, and robbing O'Brien of about the most screen-time he got this season. I like Ezri, but this episode doesn't do a thing to me.