The top 3 are:
Raichu- base 110 speed. Can learn Agility and Electro Ball as a Pikachu at level 18(Electro Ball) and 37(Agility)
Galvantula- base 108 speed. Can learn Agility and Electro Ball at level 29(Electro Ball) and level 40(Agility)
Emolga- base 103 speed. Can learn Agility and Electro Ball at level 26(Electro Ball) and level 46(Agility). Emolga also gets the Hidden Ability Motor Drive in which boosts your Speed by one stage when hit with an Electric-Type move. It's similar to Lightningrod, except Motor Drive doesn't draw in all Electric-Type Moves being used.
Hope I helped!
Source (just searched Electro Ball and Agility and figured out what the fastest Pokemon were)