the history of rugrats is fascinating to me. it starts off so strong and it just gets worse in segments.
the first era of the show is season 1. this is before angelica really came into her own as the antagonist of the group. season 1 was about as close to the original idea that arlene klasky had for the show than anything that came since. remember the episode where they're in the theater, and there's almost no dialogue and it's just animation of babies running amok? that was what rugrats was supposed to be.
once angelica was introduced as the antagonist, everything changed. the focus was less on the animation and more on the writing. she was a brat that you could love to hate, and there were a lot (a lot) of jokes in there that weren't really meant for kids. klasky clashed with the writers over angelica in particular. she didn't want to make angelica a bully, but the writers won out in the end for the first production run of the show. 1991-1994 is the classic era of the show. the passover episode marks the end of it. during the first production run, things got so bad that gabor csupo acted as a mediator between klasky and the writing staff, and most of them left when the show went on hiatus.
when the show came back in 1997, the third big era of the show began, and it's marked with a change of the animation style, increased color saturation, increased reliance on mark mothersbaugh's music and sound effects, and generally less edgy writing. one of the problems klasky had with the original show was that babies did not act like babies. without a writing staff to fight her for 65 episodes, i have to think this was more to her liking. angelica was still a brat, but she was toned down. episodes would end with her learning a lesson instead of being defeated somehow. in general, the writing was a lot less subtle, with characters sometimes outright stating how they feel (and that makes me feel angry).
the next period of the show was when dil was added. this was in 1999, right after the movie, and this is when it started explicitly appealing towards the nick jr. crowd. i stopped watching the show at this point, but i managed to catch some episodes as part of the '90s are all that' and... it's not very good. dil as a character is basically a prop in these episodes, and most episodes deal with the baby (who is too young to communicate with the other babies and toddlers) as a thing to find, since dil is either getting lost or needs to be rescued. the majority of his on-screen time is making unintelligible noises or doing gross baby stuff.
the last era of the show was after the second movie when chuckie's dad marries a french japanese woman and chuckie gets an adopted sister. i haven't seen this part of the show aside from the all growed up episode that aired as the 10th anniversary show- which also serves as a series finale of sorts to rugrats and all grown up since it takes place after that show, i believe. can't really judge the entire era from one episode, but it seems like they got the extreme color saturation under control.
i think it's also fair to include all grown up as part of the series since it serves as a sequel to rugrats. for whatever reason, my sister and a friend and i sat through every episode. to be fair to all grown up, some jokes do land, and there is what i would call one good episode in the whole bunch. but if rugrats kept getting worse after the second era more and more, this was the ultimate culmination of it- and i think it's why we couldn't stop watching. in rugrats, you sorta had a hierarchy- tommy is the leader, chuckie is the cowardly second-in-command, and phil and lil the gruntwork guys who just kinda tag along (and angelica is the antagonist, of course).
in all grown up, there's no hierarchy. the show actually focuses a lot on chuckie a lot more. the characters are all hugely different. i understand that you might not be the same person you were when you were a fictional baby and a fictional preteen, but a lot of the personality traits that defined them were replaced by things they did. tommy went from being the headstrong brave one to kind of a douche filmmaker. dil is a weirdo obsessed with the paranormal- especially aliens. lil has become a normal person while phil is still as weird as he always was (which always bugged me- since the point of lil was that it was okay she was weird and instead they make her like the other girls on the show). susie goes from being a defender of the group to wanting to sing (lots of episodes are devoted to her singing). chuckie gets something of an upgrade and goes from being a coward to being kinda ambitious and eager to try new things.
angelica gets it the worst though. she's a spoiled teen, but never really acts like it. instead of being something of a bully like what was hinted in all growed up, she's more embarrassed to be around the younger guys. it would have been the easiest thing in the world to make her an antagonist who could terrorize the younger group, but she instead just wants to fit in with the popular girls and tries oh-so-hard but just can't. the one good episode i mentioned actually deals with her as a spoiled teen and learning responsibility, and it's out of place for the series, but it's okay because the series sucks.
without an antagonist to sort of anchor the show, it wanders aimlessly and a lot of episodes spend their time on character interactions with each other- which is not bad, but it usually deals with them getting upset with one another and the problem resolving itself at the end of the episode. there's also this 'wah-waaaaaah' effect that mothersbaugh uses, and it will happen at least once per episode.
you can kind of pinpoint when the simpsons went bad, and maybe separate it out into eras too, but this show got progressively worse much quicker and never recovered.