For any Pokemon ever?
Legitimately,
Ninjask @ Choice Scarf
Ability: Speed Boost
EVs: 252 Atk, 252 Speed, 4 HP
Nature: Jolly (+Spe, -Sp. Atk)
Ninjask's highest speed at L100 is 460. At full Speed Boost, which is 6 stages, it'll be 460 x 4 = 1840. Plus Choice Scarf, which is x1.5, it will be 1840 x 1.5 = 2760
As for non-legit fastest Pokemon, I guess you could hack Speed Boost onto a Deoxys-Speed.
Highest Speed at L100 = 504
504 x 4 = 2012
2012 x 1.5 = 3018