There are only three Pokemon that fits the description of fast and strong, Rotom, Raichu and Jolteon. The other electric type that is strong is Electivire but it requires trade while another strong one is Magnezone but is slow as hell so these three are the only ones that fit the description
Rotom is the easiest one to get. In the mansion in the eterna forest, you can catch this at 20:00-21:00 and not on any other times. It has a strong dual stabs in electric ghost and a very useful ability in levitate and it's stats aren't too bad with ninety five in special attack and ninety one in speed
The second strongest and fastest is Raichu. This one is a bit tricky to get since you can only catch Pikachu in the trophy garden while you need to explore the solaceon ruins to find the earliest thunderstone. However, it is weaker than Rotom but much faster
Finally, the electric type to put both Rotom and Raichu to shame is Jolteon. It is much faster at a hundred thirty speed and much stronger with a hundred and ten special attack. This one is probably the best option because you can get a free Eevee in hearthome city but like with Raichu, you can only evolve it with a thunderstone
Source: experience and this. Here is also a video that shows you where to find the thunderstone
