Two decks built on the format oldest currency: legendary creatures, and casting spells of every color.
Dominaria United is dense with legends, and the four color deck counts them directly. Playing legendary creatures stops being flavor and starts being a deck building constraint that pays out.
Converge scales an effect with the number of colors of mana spent to cast the spell. It rewards the five color deck for building an honest rainbow mana base.
Domain counts basic land types among the lands you control. Dominaria United brought it back, and it pushes the same direction as converge.
Dominaria United came out ahead of the Phyrexian invasion arc and leaned hard on the plane history. The Commander decks follow that: one is about legendary creatures across Magic's past, the other about casting everything at once.
Both decks are greedy with mana on purpose. Converge and domain make the mana base part of the payoff instead of just a cost, which is a fair trade in a format where you have plenty of turns to find your lands.
Neither deck is fast. What they are is flexible: nearly every strong legendary creature in your collection is a candidate for the four color deck, and nearly every multicolor bomb fits the five color one. That makes them among the easiest precons to slowly rebuild into something personal.
Painbow is the better first deck if you want to play all five colors and see what sticks. Legends' Legacy rewards a bigger collection, since every strong legend you own is a possible swap, and Dihada is a genuinely good protection engine once the deck has targets worth protecting.
Dihada, Binder of Wills · Shanid, Sleepers' Scourge
A four-color legendary-matters deck led by Dihada, Binder of Wills, play iconic legendary creatures from Magic's history, protect them with Dihada's vigilance and lifelink, and drain the table with legendary creature death triggers.
Jared Carthalion
A five-color goodstuff deck led by Jared Carthalion, cast multicolor spells to distribute +1/+1 counters, build Jared into a massive threat, and win through the raw power of five-color mana access.