An episode can the weakest (or worst) in several different ways, of course. Not going to rack my brain trying to come up with 30 of them though.
Shades of Grey - this one is the worst in that it's not even an attempt at writing a decent episode, IMHO. All the others supposedly were, even if they horribly failed in some aspects. That's why this one takes top spot overall, IMHO.
The last outpost - The cheesiest, even though it's really only the last part (on the planet). The first part (on the ship) is quite decent.
Code of honor - The worst in being an offensive episode in terms of racial stereotyping.
Symbiosis - The worst in being preachy. Drugs are bad, kids!
Angel one - The worst in feeling very forced (to me) in attempting the tired 'what if women were as dominant as men used to be and behaved like it' trope.
Datalore - The worst in being so explicitly targeted at (pre-)teens only. Dumbs down the crew to an incredible degree, only to make Wesley the 'hero' who is the only one that sees the moustache twirling Lore for who he is. Lore inadvertently showing his 'advanced' knowledge by spouting Pythagoras. And so on. Ugh.
The outrageous Okona - Worst 'humor'.
Sub Rosa - Worst softporn attempt, and that genre generally doesn't mix well with storytelling. Also the idea that Beverley, otherwise a highly intelligent and capable medical officer, is so easily swayed, but OK, in that respect it's really no different from many other 'alien influence' episodes.
Then, there's those episodes that aren't outstandlingly bad per se, but that I still hate watching, for some reason. Amongst those are Dark Page, Imaginary Friend and Violations. When they're over I breathe a sigh of relief.