What makes Ash's Pikachu special is:

-He doesn't want to evolve.

-His thunderbots hit even ground-types. (Kanto and Johto only)

-Can tie with legendaries as well as lose to untrained pokemon.

-He didn't need to die and be born again as a Pichu to learn Volt Tackle.

-Yet he forgets his signature move for a mainstream Electro-ball.