Worst? There are so many...
Data in "The Quality of Life" is reduced to a caricature in a paint-by-numbers plot.
Geordi is ageist in "Relics" and while Scotty may be eager but he would still not indiscriminately press buttons like that. "Relics" is a two-for-one blue light special in that regard.
While many might cite Troi episodes where she spouts "Captain, he's hiding the obvious", I'd wager "Face of the Enemy" is worse for taking the opposite extreme. The ideal poker player cold reader can't read the minds and struggles to get into a personality type that is the polar opposite and isn't found out for the bulk of the episode? Granted, season six is loaded with this sort of contrived paint-by-numbers plotting, in ways that make the complaints of "The Way to Eden" (TOS) where actors said their characters were altered for the sake of the plot seem minuscule by comparison...
Spock wasn't well-used in "Unification".
Q was turned into a series of cheap gags in VOY, though to be fair the tattoo size joke was admittedly chuckle-inducing. That aside, Q and the Continuum did get assassinated. VOY also did the same with the Borg, even before "Unimatrix Zero" and its lame "let's assimilate the captain, engineer, and science officer - only with blue lasers this time - and even give big big hints that all this won't be shocking because we'll subvert the 'how do we get them out' by nudging it in part one too."
Picard did an impressive 180 regarding the same issue "Journey's End" and "Insurrection", for absolutely no reason. But Picard was inconsistent and mopey with the prime directive in the 7 year run as well. Stewart's acting makes it all seem like treacle by comparison.
Kirk was misused in "Generations", solely so TNG could get the movie spotlight, and after how great ST6 was, but that was from a movie and not a TV episode...
As much as I love "All our Yesterdays", it's bizarre that only Spock devolves due to the time travel aspect. Kirk and McCoy, who were just as unprocessed, didn't devolve. The script's ideas were genuinely great and especially for how late in the show's run it was, but it all got overlooked - or they wanted Spock to be the butt of a joke, which is not atypical for season 3, where he is often poorly scripted. Such as in "That Which Survives", which has his cadence completely off, if not outright wooden and caricature - and may have been the inspiration for Sheldon Cooper for all we know. Having him also babble to Droxine his mating habits in "The Cloud Minders" despite a season earlier being oh-so-shy about boinking in "Amok Time" -- yes, fans have tried to find ways to make it work. Less successful than that was when Spock of all people didn't notice the differences between Bele and Lokai that Kirk and everyone else had -- again, for the sake of typical season 3 plotting as opposed to being true to the character, thus causing another proverbial assassination. Star Trek III proved a Vulcan can have more than one life so it retroactively explains all the assassinations Spock got... Oddly, how the Platonians abused him and the others is far more in-character and while working with the script, as opposed to the other way around.
Star Trek 6, for all its positives, has Communications Officer and xenolinguistics expert Uhura reduced to the butt of a joke in not knowing Klingon and having to read (incorrectly) out of a book. But was it a funny scene or what?
Star Trek 5 needlessly made everyone the butt of jokes, some more than others, regardless of the good points and moments that script had... thank Star Trek 4 and its use of comedy for that...
Data in the TNG movies had his emotion chip fused and not removable, but by the movie's end he wrote his own subroutine to control it. By the next movie he made an on/off switch for it. After that it could be removed. Then finally he has none, ostensibly unmentioned because the movies couldn't make up their minds in terms of how to assassinate Data this time 'round, or to do it seriously or as the butt of a joke that made "Mister Tricorder" genuinely funny by comparison.
Troi is made drunk in First Contact as the butt of a joke... Cochrane in the same movie becomes an allegory for Gene Roddenberry in the same movie... and the movie thought it was being clever by doing a pee joke too... yes, there's nothing more thrilling in a sci-fi adventure than to recognize characters need to take potty breaks as opposed to doing more science-fictioney things yet this isn't "Futurama" or any other conscious parody, where said jokes tend not to be used for the character assassination (and/or flanderization) trope.
Of course, Picard becomes "John McClane in space" for the final three TNG movies as well*, complete with all those self-destruct buttons sold next to the lollipops in the candy store, so rest assured that when people say the ship is a character, the sefl-destruct button is the ultimate assassination right there too...
* like or dislike, the PICARD sequel series reels him back in and that's a good thing!
And everyone in "Nemesis" is just a caricature anyway.