Question

If I get the fastest pokemon, max out its speed EVs, get perfect IVs, and train it to level 100, what will its speed be?

Answer 1

Deoxys in its speed form has the highest speed ( a base of 180)
It can be as high as 504.

If you don't count all of deoxys' forms, the next highest is ninjask, with a base of 160, his can be as high as 460.

Answer 2

Legendary speed deoxys 504

non-legendary ninjask 460