Alolan Raichu @ Wise Glasses/Focus Stash/Aloraichium Z
Ability - Surge Surfer
EV's - 4 HP / 252 SpA / 252 Spd
Modest Nature (+SpA, -Atk)
- Thunderbolt (STAB, flying & water coverage, plus paralysis chance)
- Psychic (STAB, poison & fighting coverage)
- Surf (Important coverage for ground, plus fire & rock, & decent neutral coverage)
- Calm Mind or Nasty Plot (Setup move) or Signal Beam (Important coverage for dark, plus grass & psychic)
Added: The items that you give will be dependent on what moves you decide to use. If you opt to go with Signal Beam as your fourth move, use either Wise Glasses or Aloraichium Z. You could use Focus Stash, but I personally wouldn't use it. If you go with Calm Mind or Nasty Plot, using Focus Stash is mandatory if you want to set-up. Due to Alolan Raichu's squishiness, being able to set-up without instantly getting OHKO'd is very important.
Edit: Sword and Shield has made this possible now to any Alolan Raichu thanks to TRs (technical records) and also made make it possible to use Nasty Plot as well. Aloraichium Z is only for SM and USUM. However, Signal Beam is deprecated in this generation.
In Ultra Sun and Moon you can obtain a pikachu with the move surf from the man in the surf association in Heahea City after claiming the No. 1 spot on every island's mantine surfing spot, which is pretty funny. The man also has a pikachu that knows fly.