Brainstorm gets a lot of hype as the most skill-testing card in Magic, but for my money, that title goes to Gifts Ungiven. When you cast a Brainstorm, you're only evaluating the quality of the cards in your hand, but to properly cast Gifts Ungiven, you have to evaluate every unique card in your deck, and every well-built Gifts deck has a lot more unique cards than others to leverage their key card.
Since the arrival of Unburial Rites and the trick of Gifts Ungiven as double Entomb, much of the card's complexity has been lost, since it sets up that combo more often than not, but this Sultai list eschews that line for a more classic, control-style Gifts shell.
There is still a combo to set up, Thopter Foundry and Sword of the Meek, but it's a slower combo than we're used to. Academy Ruins and Crucible of Worlds are there to ensure that you can Gifts for the combo in a deterministic way while also being solid value cards with your utility lands and trinkets.
Trinket Mage and Snapcaster Mage are the next most important cards because they give you redundancy for your Gifts Ungiven piles. The sideboard has additional copies of "trinkets" so you can put one of them next to the Mage in a pile and get the card you want or force your opponent to give you the other two. That's the kind of choice you want to generate with Gifts Ungiven, because typically the second- and third-best cards in your deck will get the job done.
Snapcaster Mage does the same thing, but with your removal and discard spells, and unlike Trinket Mage, it doesn't require you have another copy of the card you want to cast. If they give you Snapcaster Mage, you can simply flashback the card that goes to your graveyard. Easy-peasy.
Of course, there are also times when you need to cast Gifts Ungiven proactively to find a threat or to accrue value in an attrition game, and sussing out what you're trying to achieve with any given pile is critical. These decks can be frustrating to play because you can almost always look back on a decision that may have cost you the game, though it's rarely clear what was correct in the moment.
But the games where you find off-the-wall piles that do exactly what you need are among the most rewarding in Magic.