All three starters of a generation are now fully evolved in the main cast!
Yeah, didn't think about that. I think that deserves a bit of a deeper look:
GEN I/KANTO: 
* Ash got all 3 Starters.
* Only Charizard fully evolved, Bulbasaur & Squirtle never evolved.
* Bulbasaur was the only one given a reason why it doesn't want to evolve; it just prefers to be a Bulbasaur.
* Squirtle was never given a reason (or even an episode implying it could evolved & rejected it), however it could be inferred it has done so because its the leader of the Squirtle Squad and has a rivalry with a Wartortle firefighting brigade.
GEN II/JOHTO: 
* Ash got all 3 Starters.
* Bayleef evolved to midstage and stopped with no reason given. It could be possible that, due to Bayleef having a crush on Ash and it greets him by tackling him to the ground, its choosing not to evolve to the even bigger Meganium so it doesn't completely crush him.
* Quilava took till the end of SINNOH to evolve. As a Cyndaquil it was very laidback, Bulbapedia described it as the "baby" of the Johto group, so its evolution was more treated as it growing up as it became more active as a Quilava; sadly by then its time in the spotlight was over so why it only got to midstage.
* Totodile just is. It never got an episode about it able to evolve & rejecting it. But unlike Squirtle where you can infer a reason, for Totodile there's nothing. Totodile is a fun-loving Pokemon that loves to dance, and I think because of that the reason it didn't evolve is for a meta reason: Totodile dancing is cute, Croconaw dancing would look strange (I could even see Bayleef evolving made them realize how the characteristics they gave the Starters didn't translate over well to their evolutions so, while too late for Bayleef, they can at least keep the other two from evolving).
GEN III/HOENN: 
* Ash only got Sceptile and was the first of the Hoenn Starters to fully evolve. Makes sense, Ash is battling with it more often than the other Starters with their trainer.
* May got Blaziken. As a Coordinator May is Pokemon Contest focused, and while there's battling in Contests and I'm sure in the anime's world the Pokemon gains experience doing performances, because Contests is less about raw power they were able to take their time evolving Torchic all the way to Blaziken which didn't happen until the end of the Battle Frontier arc (after May's arc during that season was done). It evolving to Blaziken more felt like a final sendoff gift for May as she later left to compete in Johto Pokemon Contests.
* BROCK WHY! Brock got Marshtomp and no reason given why it didn't go all the way to Swampert. Now, as a Breeder, Brock didn't battle that often so I guess you could say that could have stumped Marshtomp's growth. However, because it was more a support Pokemon than battler, it was used more often for comic relief. As a Mudkip it was sort of a "straight man" to the groups shenanigans and as a Marshtomp noted to have picked up Brock's flirtatious quirk (though aside the one example Bulbapedia gave on its page I can't think of it doing that often, at least not as much as Brock).
GEN IV/SINNOH: 
* Ash got Torterra and Infernape.
* Dawn got Piplup and it got an entire episode around it not evolving, Dawn even being given an Everstone for Piplup. Piplup wanted to stay the way it was because its happy the way things are as long as its with Dawn. However I also have a feeling there was a meta reason: Piplup became a SUPER popular Pokemon, so being just as much as a commercial product decided it was best to keep Dawn's Piplup as it was (and this isn't going to be the last time this is likely a reason); luckily they had also given Kenny a Prinplup (which later evolved to an Empoleon) and Barry an Empoleon; said two trainers Pokemon also appearing in that episode and was part of Team Rocket's scheme to capture the entire Piplup evolutionary family.
GEN V/UNOVA: 
* Ash got all 3 starters.
* Pignite is the only one to evolve but only to its midstage, no reason given to why it stopped there. Especially confusing as Tepig evolving to Pignite was an important moment for it: Tepig had abandonment issues because its previous trainer thought it was weak. Because of this Tepig kept pushing itself during battles wanting to prove its worth to Ash. When facing its old trainer who had an Emboar, upon realizing that Ash and the other Pokemon cared about it and not its strength, it evolved to Pignite. In addition later on when Ash got Charizard back, Pignite looked up to it as a role model. You could maybe say its old trainer using an Emboar maybe made Pignite decide to stay as it was, but I feel that's a bit of a stretch. Maybe the writers, who had decided not to evolve Snivy nor Oshawott, thought it would look even further out-of-place if it had fully evolved. Luckily they had shown Bianca had a Pignite so it fully evolved to an Emboar by the Pokemon League.
* So, remember when I said they likely didn't want to evolve Piplup becuase it got super popular? Yeah, same thing happened to Snivy almost immediately and eventually later Oshawott also got to that elevasion of popularity. In-show, a reason for Snivy to not evolve may have been because it sorta had a rivalry with Trip's Servine (which later evolved to Serperior; and since he showed off those it was easy to keep Ash's Snivy as is); though they only met & battled once and Snivy never battled Serperior so if this was a reason it looked to maybe be a dropped plot point; otherwise Snivy never got an episode about it able to evolve & rejecting it.
* Oshawott too never got an episode about it able to evolve & rejecting it, though it did get a more definitive episode against its evolution WAY later in the Unova arc. Note throughout the series a few Dewott did appear, notably Burgundy had one. And Cameron was revealed to have a Samurott so Ash faced one in the League (though Oshawott never got to battle it). During the Decolore Islands arc, as if by now it wasn't clear Oshawott wasn't evolving, they visit Scalchop Island just in time for the annual Scalchop King competition. On the island they met a a rival trainer & his Dewott named Ceaser and Oshawott fell in love with the reigning Scalchop Queen, an Oshawott named Osharina. Though Oshawott would win and become Scalchop King, Osharina was in love with Ceaser so heartbroken it gave up the title (don't worry, later on the boat to the next island Oshawott fell in love with a Minccino; Ash & co. laughing at the quick turnaround). Still, there was a bit of "a pre-evolution can't beat its evolution" theme so with Oshawott winning it showed it didn't need to evolve.
GEN VI/KALOS: 
*
* Ash only had Greninja, Serena got Delphox, and Clemont got Chespin.
* Chespin never got an episode about it able to evolve & rejecting it. Since Clemont filled the "experienced mentor" role like Brock & Cilan before him, him and his Pokemon were regulated to support roles most of the time. While Chespin battled, of the Kalos Starters the group had it was more there for comic relief, being hyperactive to a fault and gluttenous. It had one conflict with a character-of-the-day Quilladin that fell in love with Serena's Eevee, but that was more about protecting its friend and Serena's Panchem was equally protective. It also had interactions with a Chesnaught though the episode was more about helping the character-of-the-day with restoring a forest that became an arid wasteland.
* So you may have noticed an asterick next to Serena's Delphox. That's because, while it's now a Delphox, during the Kalos arc it didn't evolve past being a Braixen. Braixen had become a popular Pokemon, at least more popular than Delphox, so there's that meta reason. Another meta reason could be Serena's rival Aria also had a Braixen that later evolved to a Delphox, so keeping it one stage behind created a greater challenge for Serena and Fennekin/Braixen to overcome. Otherwise after the Kalos arc the next time we saw Serena was in Journeys and Braixen had already evolved into a Delphox.
GEN VII/ALOLA: 
* Ash had both Rowlet and Incineroar, Lana had Primarina.
* Guess which Pokemon became popular? That's right, Rowlet. Not to mention Alola arc was an entirely different anime compared to previous arcs, becoming more of a school slice-of-life thus there were less battles and more hijinx adventures. In addition Rowlet was a lazy Pokemon (a joke with it being an owl Pokemon in a sunny region) which preferred to nap than battle, Ash often having to yell it to wake up when he sent it in. It would eventually develop a rivalry with Hau's Dartrix which would later evolve into a Decidueye, but that's not why it chose not to evolve. Instead it sorta decided not to evolve by becoming attached to an Everstone which it keeps in its gizzard so that it can use the move Seed Bomb. COMEDY.
GEN VIII/GALAR: 
* Goh got all 3 Starters; though initially Grookey spent some time as a Team Rocket Pokemon.
* Grookey was more of comedy relief Pokemon and it's character development mostly ended once it left Team Rocket to join Goh's team. It had one episode where it helped a Thwackey which was part of a pack of Thwackey and Grookey led by a Rillaboom. By the end of the episode, though Goh gave it the choice of joining the pack, Grookey decided to stay with Goh. Though It never got an episode about it able to evolve & rejecting it, it can maybe be inferred it was much like Dawn's Piplup and liked how things are with Goh so sees no reason to evolve.
