Pikachu first uses Thunder Shock in "Pokemon, I choose you!" (the first episode).
He first uses Thunderbolt in episode 5 "Showdown in Pewter City".
He uses Agility and Quick "Attack in Electric Shock Showdown" (episode 14).
He first uses Thunder in episode 22 "Abra and the Psychic Showdown"
And Double-Edge in ep 79 "Friend or Foe alike"
So in Kanto Pikachu used 6 different moves on different occasions. Favoring Thunderbolt, Quick Attack, Agility, and Thunder Shock for the most part.
Then he uses Tackle in episode 118 (which is not a move Pikachu can learn in the games).
And he uses Leer in episode 150 (also not a move he can legally learn in the games).
Overall in the first series (seasons 1-5) he uses Thunder Shock, Thunder Bolt, Agility, Quick Attack, Thunder, Double-Edge, Leer, and Tackle.
He learns Iron Tail in episode 287/AG013.
And he learns Volt Tackle in episode 424/AG150.
So in gen 3 (seasons 6-9) he only learns 2 new moves. And he uses Iron Tail and Volt Tackle frequently.
He doesn't learn any new moves in the Diamond and Pearl series.
And then he learns Electro Ball in episode 677/BW020.
He doesn't learn any new moves in the XY seasons.
And while he has dropped a couple moves like Thunder and Thunder Shock, he still generally uses most of the moves in this list occasionally.
So, pretty much Thunderbolt, Agility, Quick Attack, Thunder/Thunder Shock for the first series. And Thunderbolt, Quick Attack, Agility, and Iron Tail/Volt Tackle in gen 3. And the same thing in gen 4, and then in gen 5 Volt Tackle seems to be replaced with Electro Ball for the majority of his battles (but I believe he still uses Volt Tackle sometimes).
I believe his most frequent attacks used currently are Iron Tail, Thunderbolt, Quick Attack, and Electro Ball.
Source: This Bulbapedia page