Captain Pikachu isn't bad by any means, but let's face it, Ash's Pikachu codified what Pikachu is within the franchise. Nearly every version of the species feels like some variation of him. He's like Team Rocket's Meowth, he's just instantly who you think of when talking about that Pokemon.
Besides that, I think Ash's Pikachu is a reasonably nuanced character as a result of all the screentime he's had, and his characterisation not altering as drastically at whim of the show's direction like his trainer or other characters. I like his subtle growth from a scrappy brat, to a big brother type, to an altruistic veteran as the show goes on, while still retaining that goofy playful critter vibe to him.