At least pertaining to the "classic" run (1963-1989):

Philip Hinchcliffe and Robert Holmes (https://www.doctorwhotv.co.uk/hinchcliffe-holmes-doctor-whos-golden-era-74963.htm) to Graham Williams (http://www.pagefillers.com/dwrg/williams.htm): The show goes from gothic Hammer (https://nerdist.com/article/time-lord-gothic-how-doctor-who-does-classic-horror/) pastiche with an emphasis on violence (https://books.google.com/books?id=ZdyJAwAAQBAJ&pg=PA61&lpg=PA61&dq=emphasis+on+violence+dr.+who&source=bl&ots=IX_bsi6pZ4&sig=ACfU3U0mcvo9HB4i9ymwxIlSYG_Ob1Ib0A&hl=en&sa=X&ved=2ahUKEwiN5Injgc_1AhU5lGoFHYx0ClAQ6AF6BAgoEAM) to a much more family-friendly vibe (https://www.quora.com/Did-Doctor-Who-become-too-silly-when-Graham-Williams-was-a-producer) and energy, with Tom Baker pretty much allowed to play the role his way unrestrained and able to do anything he wants. Douglas Adams (https://thedoctorwhocompanion.com/2017/03/08/douglas-adams-and-doctor-who-a-perfect-fit/) comes in half-way and shows the program what it would look like if it were written (https://storiesbywilliams.com/2013/03/12/remembering-douglas-adams/) by a genius...but, like (https://www.quora.com/Why-do-people-think-Doctor-Who-went-downhill-once-Steven-Moffat-became-showrunner-He-seems-to-me-to-have-a-much-more-organically-complex-and-nuanced-style-of-writing-than-the-previous-showrunner-Russell-T-Davies) Stephen Moffat (https://www.quora.com/Why-do-some-Doctor-Who-fans-say-Steve-Moffat-is-ruining-Doctor-Who), he made for a so-so behind the scenes kitman (https://www.merriam-webster.com/dictionary/kitman#:~:text=Definition%20of%20kitman,matched%20hardware%20for%20each%20car) as script editor.

Graham Williams (https://if-john-nathanturner-never-produced-doctor-who.fandom.com/wiki/Graham_Williams) to John-Nathan Turner (https://www.reddit.com/r/gallifrey/comments/7splfw/is_john_nathanturner_pretty_universally_hated_by/): JNT became the first real 'public face' (https://tvtropes.org/pmwiki/pmwiki.php/Creator/JohnNathanTurner) of Doctor Who production (https://www.denofgeek.com/tv/top-10-doctor-who-producers-part-one-2/), ahead of his time in many ways and making the role of producer just as much a showbiz thing as any actor in the part, but his eagerness (https://web.archive.org/web/20061031135101/http://www.jumptheshark.com/d/doctorwho.htm) to shock and surprise (https://www.reddit.com/r/gallifrey/comments/rdatb2/what_do_we_think_of_john_nathanturner/) audiences and fans led to Doctors being unceremoniously fired (https://olddoctorwho.com/why-was-colin-baker-fired/) (Colin Baker (https://screenrant.com/sixth-doctor-who-colin-baker-actor-fired-reason/)) or left dissatisfied (https://tvtropes.org/pmwiki/pmwiki.php/Trivia/DoctorWhoS21E1WarriorsOfTheDeep) with the part (Peter Davison (https://www.looper.com/248434/the-real-reason-peter-davison-left-doctor-who/)), frustrated (https://www.digitalspy.com/tv/cult/a860590/tom-baker-doctor-who-john-nathan-turner-feud/) leads (https://olddoctorwho.com/producers/john-nathan-turner/) (Tom Baker (https://www.youtube.com/watch?v=kVKMvJdfILA)) and script editors (Eric Saward (https://www.reddit.com/r/gallifrey/comments/krmue7/from_where_did_the_claim_that_eric_saward/)), several miscast (https://whatculture.com/tv/every-classic-doctor-who-companion-ranked-from-worst-to-best-2) companions (https://www.denofgeek.com/tv/doctor-who-ranking-the-companion-departures/), and storylines (https://www.youtube.com/watch?v=7h4zGWsy4do) penned (https://doctorwho.org.nz/archive/tsv3/season22.html) by relative newcomers (https://tvtropes.org/pmwiki/pmwiki.php/AudienceAlienatingEra/LiveActionTV) not just to Doctor Who, but to science-fiction writing in general. Fans' frustrations (https://burrunjor.com/2015/05/11/why-john-nathan-turner-was-good-for-doctor-who/) grew as the viewing figures diminished (https://whatculture.com/tv/10-times-doctor-who-jumped-the-shark-and-nuked-the-fridge?page=5). A brief return to form for Seasons 25 (https://www.youtube.com/watch?v=crcD3KiVtP8) and 26 (https://www.youtube.com/watch?v=eXszlhaF704) could not (https://whatculture.com/tv/7-moments-that-almost-killed-doctor-who?page=4) save it, and the program fizzled out (https://www.cnet.com/news/fantastic-doctor-who-returned-to-tv-15-years-ago-today/) in 1989 (https://anglotopia.net/british-history/who-history-the-final-days-what-led-to-the-death-of-classic-who-in-1989/) for the majority (https://www.youtube.com/watch?v=XgxC0Y9h3ZI) of the 1990s.


Being a Long Runner with a huge fanbase, Doctor Who (https://tvtropes.org/pmwiki/pmwiki.php/YMMV/DoctorWho) seems to be called on this one (https://tvtropes.org/pmwiki/pmwiki.php/SeasonalRot/LiveActionTV) with every season, with symptoms ranging from cast changes to shifts in direction to question lapels appearing.

While not a severe case of this trope, the last full seasons for both the First and Second Doctors (Seasons 3 and 6 respectively) are felt to be a slight step down after two very solid seasons each. While both Seasons contain great stories, they also contain some of their weakest stories of their tenure in both seasons.
Season 11, the Third Doctor's last season, is often considered his weakest. It did introduce one of the best-loved companions, Sarah Jane Smith, in the well-received first story "The Time Warrior, but "Invasion of the Dinosaurs" is a good story with awful special effects, "Death to the Daleks" and "The Monster of Peladon" contain a lot of Recycled Plot and "Planet of the Spiders" feels quite padded. It didn't help that the Master's actor had suddenly died, scrapping the original plans for an emotional sendoff to the Master and the Doctor.
Many fans found Season 17 (the season Douglas Adams script edited) the weakest of the Graham Williams era. Yes, "City of Death" is almost universally considered to be one of the best stories of all time, but it doesn't make up for the despised "Destiny of the Daleks", "The Horns of Nimon", the innuendo-laden "The Creature from the Pit", the Anvilicious "Nightmare of Eden", the ridiculous "The Horns of Nimon or the fact that the entire season was cut short by a poorly-timed crew workers strike.
Conversely, Season 18 is often considered an overreaction that went too far the other way. New production team producer John Nathan-Turner and script editor Christopher H. Bidmead declared their intention to make the show "less silly" and produced a season rather dour and humourless, more interested in technical and philosophical matters than an eccentric hero fighting monsters. Tom Baker often looked a shadow of his former self, forced to play the role in a way he disliked, and popular companions Romana and K9 were replaced with Adric. Of the seven stories, only "Full Circle" and "State of Decay" really felt like Doctor Who. Of the others, three ("The Leisure Hive", "Meglos" and "The Keeper of Traken") were good ideas realized in an esoteric, poorly paced and rather flat manner by the direction and script editing, and the other two ("Warriors' Gate" and "Logopolis") were esoteric and poorly paced to start with. Bidmead only lasted one season, after which the show largely reverted to a more familiar action-adventure style.
The Fifth Doctor's middle season (Season 20) is generally considered the weakest of his three, due to nearly every story being So Okay, It's Average and lacking any of the memorable episodes such as "Earthshock" in Peter Davison's first season, and "The Caves of Androzani" in his last.
One thing nearly everyone seems to agree on is that seasons 22 through 24 (1985-7), better known as the two seasons of the Sixth Doctor and the first season of the Seventh Doctor, were the nadir of the classic series, although different fans pick different ones of those seasons as the worst.
Season 22 saw Eric Saward going overboard with the Darker and Edgier, with the Sixth Doctor (Colin Baker) acting like an a**hole most of the time (including to his own companion), grim plots with lots of Black-and-Gray Morality and Kill 'Em All, and enough Family-Unfriendly Violence to, for the only time ever, cause the fans themselves to start getting uncomfortable. Saward was also purported to dislike Baker's performance of the Doctor and reduced his role accordingly, to the point that in "Revelation of the Daleks" the Doctor is arguably completely superfluous to events.
Season 23 had the unpopular "Trial of a Time Lord" extended arc, some very lackluster writing, and an ending that revealed the behind-the-scenes chaos the show had descended into by being almost incomprehensible, unintentionally. Both this and the previous season are also notable for bothersome amounts of Continuity Lock-Out and Continuity Porn.
Season 24 suffers from a Lighter and Softer shift that many fans considered to go too far into glitzy Camp, Sylvester McCoy playing the Doctor as an actual Ditz instead of his later, more popular performance as a world-weary Manipulative Bastard who occasionally engaged in Obfuscating Stupidity, and Keff McCulloch and his disco-aerobics brand of incidental music.
All three seasons also suffered from having two of the most widely unpopular companions in the show's history: Peri who was one of the few pre-1989 companions to genuinely be as hapless and frequently-demeaned as post-2005-only fans and lazy media commentators claim all of them were; and Mel who was just annoying and played by an actor who had an irritating public image and a lot of baggage from earlier roles.