I've been playing around with Raichu-A and I've come to the conclusion that this is the best set in OU (in my opinion):
Raichu-A
Timid/Modest Nature
Life Orb
-Thunder
-Psychic
-Focus Blast
-Nasty Plot/Grass Knot/HP Ice
Game Freak set up Raichu to be a niche destroyer. Counterpicking mainly Celesteela, Pheromosa, and Toxapex using its tools.
The Nature could go either way, really. Modest is really great for letting Raichu hit harder overall, but timid lets you outspeed +1 speed Pheromosa, which is good because Pheromosa is the definition of wallbreaker. It's situational, but it works.
Life Orb's obvious. You just hit harder. No Choice Specs because it limits you, and no Choice Scarf because you're already the fastest thing in the game with Tapu Koko. Aloraichium Z is also pretty good, but... you have to lose power overall just for the sake of ONE really powerful attack. Thunder has to go to Thunderbolt in order for the Z-move to work, and your other moves lose their extra 50% power.
Thunder hits hard, gets 1/3 chance for paralysis, and is basically 100% accuracy with Pelipper around
Psychic is of course there just for Psychic-type STAB. Psyshock could also work, but it's very situational for when it does. (It's only pretty much optimal for Chansey)
Focus Blast is there to hit Steel and Dark types hard.
Nasty Plot may be debatable, but I find it to be great. With Tapu Koko having Terrain Extender, it's a way to deal with set-ups on the opposing team. Mainly leech seed users and protectors that try to stall a turn are in this category. They stall, you Nasty Plot. You have enough power to kill Garchomp with Psychic. Most things fall to Thunder, Psychic, and Focus Blast anyways when +2 boosted. However, I strongly advise that you do not run Nasty Plot on Raichu when your Koko is running another item. Use Grass Knot or HP Ice for coverage instead