[Deck Knight]

LOL Bob is going to hate me for this by adding yet another option:

Deck Knight's case for Electric/Psychic

Passive damage weakness Check. – SR, sandstorm, spikes, even U-turn or Pursuit, whatever it might be

While Electric/Psychic doesn't have an SR weakness, it is susceptible to all forms of passive damage as well as Sandstorm, and it combines a Volt Switch resistance with a U-turn weakness, and has a weakness to Pursuit, meaning if you select a switch poorly, you can be punished.

Good overall STAB combo Check. – that means it should have both a wide coverage over the OU metagame and
STAB that cannot realistically be spammed endlessly for maximum impact – That means no dragons (oh fine if you want)

Psychic and Electric are not resisted by any single type, and they score super-effective hits against large numbers of Bulky Waters and Fighting types. Combined with Fighting coverage it effectively deals with Ferrothorn and Magnezone, two Steel-types that resist the combination, or it can have Fire coverage for Ferrothorn, Magnezone, Metagross, and Specially Defensive Jirachi. Both attacks have a type immune to them (Ground and Dark, respectively)

Lots of weaknesses and resistances Check. – It should be a risk to switch in, but that doesn't mean it should be impossible to do so

Psychic/Electric have no overlap on the type chart, so the resistances of Electric (Electric, Flying, Steel) and Psychic (Fighting, Psychic) are both represented, as well as the weaknesses (Ground, Bug, Ghost, Dark). Similarly, the super-effective coverage (Water, Flying, Fighting, Poison) hits a few key target types, but is not overwhelming..

Notably, this combination produces a resistance to the Fighting/Flying combination of Tornadus-T, while leaving CAP4 vulnerable to U-turn, so what you switch in on can significantly impact CAP4's Performance.

STAB on notable risky moves Not really. – eg. Sucker Punch, Focus Punch, anything with a negative effect in essence (though I don't really like Sucker Punch but whatever)

What Electric/Psychic offers is a STAB that can feed off of Rain but is otherwise inaccurate (Thunder) and a STAB Special attack that can break physical walls in Psyshock. It resists the two most common priority attacks, Mach Punch and Bullet Punch, but is weak to Sucker Punch. Since Fighting tends to cover it so well, a more physical variant could potentially use SubPunching very effectively as a payoff for risk.