Much like the government itself, Parks and Recreation is a show by and for the people. For 7 seasons the show made a point of humanizing the characters shown, even if they are only around for a short time. There is something very realistic about the way characters come and go on this show, and this is part of what makes it so relatable and real, even as the people of Pawnee become more and more absurd.
Even the characters with one-off appearances make a lasting impression. It's difficult to rank characters that only appear for single episodes, but the rubric used here is simple: characters are ranked based on how memorable they are, how much depth they have, and how impactful they are to the show in general.
Scott Braddock - "Kaboom" Season 6 Episode 2
After Andy hurts himself in the notorious 'pit' in season 2, the Pawnee city attorney Scott Braddock arrives to warn Leslie away from liability. He has a surprisingly small role to play in the grand scheme of things, but the impact he has is long-lasting and palpable. Scott is a major part of making sure the pit outside Anne's house gets filled in. This sets Andy and Anne off on their arcs, leading to Andy's growth and Anne's career change. The high-strung attorney is deeply funny in how much pressure he puts on Leslie. His insistence that she not it fault pushes against her moral center. Scott's depth is a little lacking, but he makes up for it by being a memorably funny counterpoint to Leslie Knope's hilarious and caring nature.
Tynnyfer - "Doppelgängers" Season 6 Episode 4
While visiting the Eagleton parks department, Leslie's team is paired off to learn from the rival city's workers. April is sent to work with Tynnyfer, the standout among them. She is the shallow and shiny counterpart to the cynical and funny April Ludgate, giving a unique foil to the lovable goth woman. What's more, Tynnyfer stands out as the worst person April has ever met. Tynnyfer is a memorable character in the same vein as Mona Lisa and Jean-Ralphio, but with a uniquely Eagleton spin. For how obsessed April is with her, Tynnyfer has remarkably little impact on April in the long term.
Elsa Clack - "Camping" Season 3 Episode 8
When the department goes on a group retreat, the audience is introduced to Elsa Clack. The proprietor of the Quiet Corn bed and breakfast is deeply memorable for her love of cats, friendly demeanor, and harsh rebuttal to rudeness. Elsa Clack doesn't appear in the episode much, but whenever she is present you can't help but focus on her. The depth of her character comes through the B&B as much as through her own charm. From the army of cats to the charming decorations, there is a strong sense of who she is through the background that she creates. Her impact is almost nothing, but there's something to be said for the interactions that Elsa makes happen throughout the retreat.
Clarence Carrington - "Summer Catalog" Season 2 Episode 20
Leslie's ion for her work is often tempered by the men who came before her. In the Summer Catalog episode, Leslie goes hiking with the past and present leaders of the Parks Department, the worst of whom is Clarence. Clarence is a rude and hostile character who is memorable for how vile he is. His constant sexism is comically overdone, providing Leslie with a superb foil.
His insistence that women belong at home seems to push Leslie to be even more involved and active, just to spite him. He doesn't provide much character depth while on-screen, but other characters fill in his history and provide a clear picture of the man. He's another character that lacks notable impact beyond background and history for why Leslie works so hard to prove herself.
Raul Alejandro Bastilla Pedro de Veloso de Morana - "Sister City" Season 2 Episode 5
In an episode mocking how little small town 'sister cities' know about each other, Pawnee is visited by the parks department of Boraqua, Venezuela. This episode is proof that Fred Armeson is a character actor like no other. His portrayal of the Venezuelan delegate Raul is among the most memorable in the entire series. Raul's constant degradation and mockery of the Pawnee Park Service is part of what makes him so memorable. He's not really a villain, but he's hilariously antagonizing the entire time he's on screen. However, his depth and impact are a little slim. There's very little depth to him, but he does have a sliver of impact. His offer to pay to fill the pit is notably tempting for Leslie, but his constant jabs keep her from taking it.
Garth Blundin - "Article Two" Season 5 Episode 19
When Leslie's underdog status was hard to continue once she was elected to city council, but the continued refusal to change or develop keeps her as a fighting force. When she attempts to make changes to the city charter, Garth Blundin stands up. Garth is a perfect example of how the city Leslie works for continues to push against her best intentions. Garth's impact is excellent, he provides a point of contention to a major project of Leslie's, and he provides a lesson in negotiation and creative problem-solving. His depth is well executed in his historian interests mixed with his Star Wars obsession. And he is solidly memorable thanks to the quips and remarks that Patton delivers pitch-perfectly - he improvises some of the best lines in all of Parks and Recreation.
Chipp McCapp - "Flu Season 2" Season 6 Episode 19
Late in season 6, Chipp McCapp (Bo Burnham) is introduced as a young celebrity singer who is slated to headline the Pawnee Unity Concert. The singer/songwriter/comedian steals almost every scene he appears in. His impact is subtle but interesting: as Leslie has discovered that she is pregnant, he provides a petulant child-star for her to dislike and develop concerns from.
Chipp doesn't even appear in that much of the episode, but still manages to leave her questioning. Chipp is not a very deep character, being a spoiled and entitled star. But the standout nature of Bo Burnham makes Chipp McCapp a deeply memorable one.
Derek - "Bowling for Votes" Season 4 Episode 13
Derek is one of the strongest one-episode characters because the episode he is in (Bowling for Votes) is focused so heavily on him. When Leslie polls townspeople for her city council campaign, he is one of the few who dislike her. Leslie learns important lessons from Derek - the importance of moving on, knowing when to take criticism seriously, and how to tell whose opinion really matters. He has a clear and important impact on Leslie's direction. Derek's memorability comes partly from how dislikable he is. His insistence that Leslie is 'pretentious' comes from nothing, and his final line is just a crass insult. However, the thing that holds Derek back from being the best character he can be is the relative shallowness of his character. Despite Leslie's full dossier on him, very little of him is shown - probably for the best
Tamara Swanson - "Ron and Tammys" Season 4 Episode 2
Tamara Swanson appears early on in season 4, when Leslie goes to her for help separating Ron from his ex-wife, Tammy 1. The original Tammy, Tammy 0, provides the perfect explanation of where the ever-present Ron Swanson comes from. Tamara Swanson is gruff, no-nonsense, and just as controlling as Tammy 1 and Tammy 2. Her impact on Ron is clear, the audience can see where he gets his love of guns, meat, and freedom. Everything that makes Ron who he is comes from her. She is also a surprisingly deep character. Tamara is a mother who cares deeply for her son, but also respects his decision to step out from under her - and Tammy 1's - thumbs. However, Tamara Swanson doesn't get as much screen time as she could have, due to much of the episode being taken up by Tammy 1.
Mayor Walt Gunderson - "Two Funerals" Season 7 Episode 11
Perhaps the most impactful, memorable, and deep character on the entire show is Walt Gunderson. Despite being referenced multiple times throughout the series, the former Mayor only appears in one episode - featuring his funeral. The indescribable Bill Murray provides the character with a fey wistfulness to the mayor's final words, and adds to the memorability with his star power and personality. Throughout the second season, the parks department is doing work at the behest of Mayor Gunderson (And his dog, Rufus), indirectly setting up many major storylines and character arcs. For character depth, he is nearly a mythical character throughout the series, but his final words are rather down to earth about his legacy and relationships.