Mamoru. Mainly because while the anime never gave him anything more than his roses and the occasional moments with his cane, the manga gave him an attack of his own that was strong enough to kill some of the villains. He still died without a fight in the Sailor Stars manga though. In his defense, so did most of the cast of the manga.
Sailor Moon herself in comparison to her manga self. In the vast majority of episodes, she spends most battles against the monsters of the week comically fleeing from their attacks or being flat-out hit by them. The other Guardians then arrive and do most of the real fighting, with Sailor Moon only getting in the finishing attacks thanks to their efforts (plus occasional help from Tuxedo Mask). This wasn't the case in the first season where Sailor Moon was fairly competent even after the others had awakened, but from season two/R onwards, she became laughably incompetent in fights.
One very good example of this is in Stars. In the Anime, the Starlights do most of the fighting against Galaxia and she overpowers them and Sailor Moon. In the manga, Galaxia and Sailor Moon are on equal footing and Sailor Moon fights her alone.
Sailor Venus starts out as the most skilled and the first sailor soldier to have her powers awakened. She beats the crap out of the bad guys in her first appearance and is incredibly competent. As the seasons pass, she becomes ditzier, is less and less effective in battle and gets a big dose of chickification. In fact, Usagi was originally the ditziest and chickiest girl in the group but her character development into the leader meant that Minako would have to take on the role as the chick. Granted, Sailor Venus in the anime was never as badass as her manga counterpart in the first place.
Most of the Soldiers get hit with this during R, as none of them are able to finish off a Cardian/Droid by themselves (with rare exceptions). They all needed Sailor Moon to land the finisher, which wouldn't be so bad if most of the time they weren't just standing around in condition to keep fighting and if it wasn't the only thing Sailor Moon did in a lot of those fights.
The monsters of the day became generally goofier as the show progressed and the Quirky Miniboss Squad of each villain faction became less and less menacing too.
Starter Villain Jadeite started off a competent threat. He had powerful minions, curb-stomped Sailor Moon during their first meeting, and actually managed to succeed in getting away with human energy in one scheme, earning Queen Beryl's compliments. But right after that last event, things began to go downhill for Jadeite. Very downhill, as he stubbornly refused to evolve his plans and strategies even when they clearly weren't working. Once he got Hoist by His Own Petard for the last time, Queen Beryl "decommissioned" him for good. The rest of the Shiten'nou avoided the trope, with Nephrite and Kunzite never ceasing to be threats (though Kunzite slips when he gets his own arc, there is more of a justified in-story reason for it and he manages to recover toward the end), and Zoisite being more bark and dirty tricks than bite until the last few episodes of his arc in which he took a more threatening turn.
Queen Beryl actually comes off as weaker than Kunzite in the anime and number of his plans end failing due to her sending the brainwashed Endymion out with him, compared to the manga where she was shown to be stronger than him. Also, she suffers a mortal wound from a flower.
The Ayakashi Sisters in the manga are murderous maniacs, while in the anime they were merely misled and are granted a chance to live free in modern day Tokyo. The manga also had them capable of killing the Sailor Guardians with ease, something they struggled with in the anime. Possibly a case of Tropes Are Not Bad since they also contributed to the other Sailor Guardians. being Demoted to Extra in that arc of the manga.
While more decayed in demeanor rather than character, the frightening Zirconia in SuperS also got a lot more frustrated and pathetic in the last half of the series, whereas in the first half she was chillingly calm and foreboding. Justified in that she had to deal with more rebellious, troublesome minions and orders from the realBig Bad, Queen Nehelenia.
