Which Strixhaven deck is best?

Sultai Ultimatum Sultai Ultimatum Is the Best (?) and It's Not Close (Green/Blue/Black Control/Ramp) In my estimation and many others, Sultai Ultimatum is the deck. It's one of the best MTG Arena Strixhaven decks. It takes up a great deal of the meta and resolving just one copy of Emergent Ultimatum can really get things going.

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You can also ask why is it called elder dragon highlander?

It's short for "Elder Dragon Highlander". Highlander because it's a singleton format (you may only play one of each card) and Elder Dragon because in it's first set of rules your commander had to be one of the original Elder Dragons. Then, can any legendary creature be a commander? When choosing a commander, you must use either a legendary creature, a planeswalker with the ability to be commander, or a pair of legendary creatures or planeswalkers that both have partner. The chosen card or pair is called the commander or general of the deck.

Correspondingly, can you have more than 100 cards in a commander deck?

How many cards are in a MTG Commander deck? A Commander deck is 100 cards and you can't have more than one of any card that isn't a basic land. That means it's a little bit harder to try and optimise than most formats, but that's also a big part of its charm.