Raiden Shogun

5★

ElectroPolearmMain Damage

TL;DR

Raiden does two jobs at once. Her Skill sits on the team chipping in Electro damage and making everyone's bursts hit harder, and her Burst puts her on the field for seven seconds of very large numbers that also refill the rest of the team's energy. Cast hers last, after everyone else has used theirs, and it hits harder for it. Energy Recharge is her damage stat, which is unusual and makes her one of the easier 5-stars to build.

Rotation

Skill (E) first, before anything else, so the burst buff is up for the whole team.
Swap through the rest of your team and use all of their bursts.
Come back, Burst (Q), and swing for the full 7 seconds.
Swap out. Everyone's energy is mostly refilled, so go straight into the next rotation.

Abilities

Skill (E)	Leaves a floating eye that hits enemies about once a second while your on-field character attacks, and buffs the whole team's burst damage. It lasts 25 seconds, long enough to cover a rotation.
Burst (Q)	She pulls out a very big sword for 7 seconds. Her attacks turn Electro and count as burst damage rather than normal attack damage, and they hand energy back to the other three characters five times over the seven seconds.
Her stacks	Every time a teammate uses a burst, Raiden quietly banks a stack. The more she has when she casts, the harder her burst hits. There is a counter, you will never need to look at it.

The one mechanic to understand

Energy Recharge is her attack stat.

On most characters you stack Energy Recharge until the burst comes back on time and then stop. On Raiden every point above 100% keeps paying, turning into Electro damage bonus and into how much energy she gives the team. So there's no such thing as wasted Energy Recharge on her, and the usual tug of war between "can I cast this" and "does it hurt" just doesn't happen. Around 220% is comfortable if she's your only Electro character, and noticeably less if she has company.

What actually matters

The two or three real decisions. Everything else is noise.

Burst last. Her stacks come from your other characters bursting, so going first throws away a big chunk of her damage. Skill first, everyone else's bursts, hers at the end.
Energy Recharge over everything. Substats, sands, weapon. It is the one stat that never stops giving on her.
Level her burst talent first. Her swings during the burst count as burst damage, so the normal attack talent does almost nothing for her. Burst, then skill, and leave normals last.
She recharges the team, not herself. Those swings top up the other three. She still has to find her own energy, which is why her skill wants to be up all the time.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge. The Catch is free from the Inazuma fishing association and adds burst damage and burst crit on top, which is the exact shape of her kit. Refine it to R5, the materials are cheap.
Artifacts	4-piece Emblem of Severed Fate. It converts Energy Recharge into burst damage, and all of her damage is burst damage.
Main stats	Attack or Energy Recharge sands, Electro goblet, Crit circlet. Take the Energy Recharge sands unless her burst is already coming back every rotation without it.

Works well with

Hydro

Pyro

Dendro

Pyro is on the list because her most famous team is a pair of Pyro characters plus a Hydro one, all of whom are hungry for the energy she hands back. Dendro is the other direction, where her Electro sets off Bloom reactions.

Don't bother

Watching the stack counter. Skill first, teammates' bursts, hers last, and the counter looks after itself.
Levelling her normal attack talent early. It barely touches her actual damage.
Worrying you have too much Energy Recharge. There isn't such a thing on her.
An Attack goblet. Electro damage wins once she has a reasonable attack base from the sands and her weapon.

Want the full theorycrafting version? Read Raiden Shogun on KeqingMains.