Regeleki is a fascinating case. With a Base 200 Speed, it dethrones Deoxys Speed as the Fastest Pokémon in the game, making it incredibly hard to outspeed at +1, and with Transister, its Electric attacks pack an incredible whallop. On the other hand, Electric is all Regeleki has. It can't even deal with Resists like Regidrago can, outside of explosion unless they are really frail and anything with an immunity is a hard stop. Regieleki has the highest highs and the lowest lows. With no Electric immunities, Regieleki can run havok, tying down targets, counter switch-ins, and generally ripping holes in the opposing team. Bring in a Ground type and watch all that go away as Regieleki flees for its life.
I Am Lightning
Regieleki
Ability: Transistor
Items: Magnet
-Thunder Cage
-Volt Switch
-Explosion
-Ancient Power/Wild Charge
Nature: Naive (+SPE, -SDEF)
EVs: 84 ATK/252 SATK/172 SPE
Regieleki hits so hard with just a Magnet, allowing it to forgo being locked into a Choice Item, while avoid the chip damage from Life Orb, which when combined with Entry Hazards and potentially Wild Charge adds up quickly. Thunder Cage is the STAB of choice, allowing Regieleki to lock an opponent down for slightly less damage than Thunderbolt allowing Regieleki to tag out without fear of pivoting on the opponent's side. Volt Switch is another amazing move, allowing Regieleki to scout switch ins and damage targets as it switches out to an appropriate counter. Explosion is Regieleki's answer to Ground types/Electric Immune abilities, hitting for a sizeable amount of damage on a large amount of targets.
Last move is up to preference. Ancient Power enables Regieleki to safely down greatly softened targets that would otherwise be immune to Electric moves, but aren't worth the boom. Wild Charge on the other hand, is an option to strike at targets who have more SDEF than DEF, allowing Regieleki to blow by sponges.
As for the EVs, Max SATK is necessary for bringing as much oomph to the Volt Switches and Thunder Cages as possible. 172 SPE EVs with a speed boosting nature is all that is necessary to outspeed a positive natured Choice Scarf Kartana (base 109), is the fastest common Scarfers and most boosters would go in OU. The remainder goes into ATK to make Explosion (and potentially Wild Charge) sting as much as possible.
Regieleki can be difficult to play with if there are ground types on the opposing team. Letting a Garchomp or Excadrill come in and Swords Dance/Rapid Spin for free may cost you the game if your checks for them are unable to handle them. Additionally, due to its fraility, Regieleki offers no defensive utility on your team and requires a free switch to get in. But Regieleki also puts a great deal of pressure on your opponent's revenge killers as they are liable to be outsped and OHKOd by Regieleki itself. Read your opponent and Regieleki is an invaluable asset.
Also, watch out for Priority.
252 Atk Choice Band Weavile Ice Shard vs. 0 HP / 0 Def Regieleki: 160-190 (53.1 - 63.1%) -- guaranteed 2HKO
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Regieleki in Grassy Terrain: 412-486 (136.8 - 161.4%) -- guaranteed OHKO
252+ Atk Guts Conkeldurr Mach Punch vs. 0 HP / 0 Def Regieleki: 198-234 (65.7 - 77.7%) -- guaranteed 2HKO
252 Atk Tough Claws Lycanroc-Dusk Accelerock vs. 0 HP / 0 Def Regieleki: 136-162 (45.1 - 53.8%) -- 41% chance to 2HKO