Question

What is the fastest Pokémon that can learn Beat Up?

Answer 1

The fastest Pokémon that can learn Beat Up is Dragapult, with a base speed of 142.

Source:
Showdown Teambuilder

Hope this helps.

Comments

Thank you for contributing to intense BRAINPOWER

Answer 2

There's a much easier method to do, than Porygon told in Pokemon Showdown. You can do:

/nds Beat Up, speed desc

Doing this will list the Pokemon that learn Beat Up, in descending order of their speed. When you do this command, Dragapult comes at top, hence it is the fastest user of Beat Up at 142.
I hope I helped!