Throughout the story we've met many trainers, who's characters were extended throughout their pokémon or vice versa.
Pikachu
Ash's ace, strongest pokémon in the world currently and best friend. Pikachu was there with Ash through it all, the ups, the downs. They all experienced it together and came out stronger. What I like about Pikachu ia that it always mentors the younger pokémon in Ash's team, inspiring them to do better and go beyond.
With his most notable feats beating Alain's Metagross and Tyranitar, Gladions Sylvally, Kukui's Tapu Koko and Leon's Cinderace and Charizard. Back in the Sinnoh league a significantly weaker pikachu even tied with Tobias's Latios. Current Pikachu would easily body every pokémon it has lost to in the past at this point.
Charmander, Charmeleon, charizard
Being abandoned as a young Charmander to being Ash's first regional ace, defeating many pseudo legendary's to an actual legendary bird during his Hoen adventures. Whenever it came on screen you knew shit was about to go down. It warmed up to Ash eventually, became a little cocky as a charmeleon and eventually completely disobedient as a charizard, the arc of that story was pretty interesting, considering Ash was so inexperienced back then, only relying on pure power and nothing else.
Greatest feats are as mentioned earlier defeating Noland's Articuno, Iris' Dragonite, Gary's Blastiose.
Hawlucha
Being a forest champion always being obsessed putting up a show to amuse the masses never stopped being entertaining on the screen, originally having a strong rivalry with a machamp before deciding to show the region what he's made off. Most notable feats are defeating Astrid's mega absol and Alain's Weavile. While not the strongest, he had the most flair and I like jojo, sooooo.
Froakie, Frogadeer, Greninja
From a battle spirited Froakie to one of Ash's most powerful pokemon of all time, albeit through a lot of lust for power and a discord between pokémon and trainer, which partly led to their downfall in the league finals. Greninja got it all. His own theme, his exclusive move, the style, the one and only bond phenomenon that elevated Ash to pro status by placing second in the Kalos league. Shadow clones in combination with water shuriken or cut proved to be too much for many, including Diantha's Gardevoir (except Alain's mega charizard X, I'll come back to that one later)
Charizard (alain)
Alain's Charizard might even have some more badass points compared to Ash's, Charizard seeks to fight strong opponents whom can mega evolve. Alain makes a deal with Team Flare's leader to absorb Mega energy to do more research on Mega pokemon, in return Alain gets to fight interesting pokémon trainers. Initially teased as an anti-hero he gets his own specials to show his endeavors as a fledged trainer.
Charizards greatest feats are defeating Ash Greninja in the Kalos League finals, defeating TEN mega evolved pokémon in a row without any rest (including Kalos Elite 4's Malva) and Leon's Rillaboom (probably the second or third most powerful non legend pokemon in the entire series) in the Masters 8 league.
Its hard to keep track of the starter pokemon I liked over the years, as it varies heavily sometimes and even skips a gen here and there. My all time fav is Kabuto though, bar none. Pikachu is a long time fan fav of course, and I liked its look by the late 1990s. Then comes some starters, Mew and mew two, and in no particular order various ghost pokemon, Seedot, That crow with a hat pokemon, and the Unowns I suppose. Good times.