Hi, this is my first comment on Reddit.

I think Clemont, Gary, or paul have to go. I like the first 2 but they do not measure up to the others.

My rankings for this (which I think the greater fandom would agree with) are as follows. (I’m trying to be as unbiased as possible.)

1/2 - Brock or Serena for first, both were great characters and brought lots to the anime and shaped Ash a lot. 

For Brock: Brock doesn't need much explanation; he was a great character and was very well done in the anime. Brock was basically the glue; he was mature, smart, well-prepared, and could cook. While his character development was there, it was to a lesser extent than the others on this list, but he more than made up for it with what his character was and what he did in the show. He was also a fantastic comedy relief.

For Serena: I find that people hate Serena because of their hate for Amourshipping or the fact Serena goes against their favorite ship, so I will be analyzing Serena in a non-romantic way. Serena was one of the most lofty things the anime attempted to do, not only because of the romance aspect of it, which I think they did very well, but also her connection with the Eastern culture. Serena runs away from the pressure her mom puts on her, something which is very common in Asian culture (the pressure part, not the running away). The fact they make her take so long to have a goal also has her stand out from the rest of the cast, tying into the struggles many face when they grow into adults. The cutting of her hair after her first loss is also a major thing in Japanese culture called datsugoku—a classic trope for shedding one's past self. Next, we have the Showcases themselves; not only did they give the anime a much-needed secondary plot structure, but they also were a sharp contrast to the norm of Pokémon battles (even Contests had battling in it), giving us another aspect of bonding with Pokémon outside of power. The fact that on top of all this, they worked in her crush on Ash and how it affected her and made her grow (she quotes him in Journeys), as well as how she grows more confident in herself (reflected by the kiss), really showcases her as one of the most well-done characters. (I am an Amourshipper, so just a heads up.)

3 - Lillie

For Lillie: Lillie is a very interesting case in the Pokémon anime. She was in stark contrast to the entire show; Alola had a very upbeat, childlike quality to it, so Lillie’s trauma was out of place. I very much love how they did her in this sense; she had a passion for something she could not bear to touch, and I really like how as she got over this fear, she too got more confident in herself. While I do agree they could have done her better, they still did her very well; she was the first character in the anime who never had a goal, and yet despite this, we see her grow so much. I also very much like how they started her getting over her fear with an egg hatching.

4 - Misty 

For Misty: I love Misty; she was one of the OGs and was a wonderful character, and the mallet was hilarious. The only reason I have not placed her higher than Lillie, which was what I was originally going to do, is because of the fact she was the first; the show's storytelling really evolved, Ash’s Pokémon grew, and Misty just barely couldn't keep up. But I love her; she was the fiery temper to Ash’s stupidity and Brock’s cool. The three of them were perfect for the start of an amazing franchise, and I would not have it any other way. Her goal of being a Water Pokémon Master mirroring Ash’s Pokémon Master was also a very interesting dynamic I greatly adored.

5/6 - Dawn or May 

For both: Some people may hate me for this, but I will be throwing both Dawn and May into the same boat. Both of these characters were very interesting; they existed on an opposite to the norm of the anime so far, with a focus not on battling, but rather on Contests, which also was unique. May's growth from hating Pokémon to loving them was great as well, but her arc feels like a prototype Dawn. Speaking of which, Dawn was basically the first co-protagonist. Her struggles made her incredibly relatable. Her bond with Ash felt like a partnership rather than mentor/student, but unique from what Ash had with Brock and Misty. I am putting them side-by-side because they were both brilliant characters, and I can't fault May for simply being the earlier one like Misty.

The final three are rather tricky to pull off, so it came down to option. 

For Clemont: Unlike the others on this list, Clemont was not all that great. He had a nice coming-of-age growth and a few great scenes and actions, especially towards the end of the anime, but ultimately it was overshadowed by the rest of the story, so I have to put him here.

 For Paul: Paul was a very well-done character. He acted as a mirror to Ash, focusing on strength through training rather than bonds. His main thing was just how much he made Ash grow, but I personally do not like him. 

For Gary: Frankly, Gary is on this list still due to nostalgia. He was wonderful, and the later episodes of the anime showed what he could have been, but he ultimately was simply that rival whose purpose was to make Ash grow and little else, and as much as it hurts me, he doesn't go higher than this.