Counterspell

Despite its age, Counte๊rspell remains versatile and reasonably-priced.

Mana Drain

Combining Counterspell's flexibility and cost with a theoretically large amount of colorless mana on your next turn makes Mana Drain one of the most feared blue cards around.

Swan Song

While it might not be as versatile as the others, Swan Song's lower cost and barely-there drawback of a 2/2 Bird in a format with 40 life cement its staple status.

An Offer You Can't Refuse

An Offer You Can't Refuse is not just fun to say, it can also handle spells of almost any type for only one blue mana. The drawback of your opponent gaining two treasures is almost always offset by the mana they had spent to cast their spell in the first place.

Fierce Guardianship

Even better than a one-mana counterspell is a free counterspell, which Fierce Guardianship will be for the majority of the game. It should be noted that the card only specifies that you control a commander, not your commander, so you can utilize its effect if you've stolen an opponent's commander.

Force of Will

Force of Will should almost never be cast by paying its mana cost, ✤and instead held as a free counဣterspell. Bear in mind that players will often play cards most detrimental to blue when the blue player(s) are out of mana sources.

Pongify

Exchanging a player's commander or game-ending threat with a more vulnerable 3/3 Ape at instant speed is more value than you could ever want at just one blue mana.

Rapid Hybridization

Also, exchanging a player's commander or game-ending threat with a more vulnerable 3/3 Frog Lizard at instant speed is more value than you could ever want at just one blue mana.

Reality Shift

Even stronger than destroying a creature is exiling it, so your opponents cannot bring it back from the graveyard. Although Real🧔ity Shift switches that exiled creature for a 2/2, it does have the very small potential to turn into a threat of its own if the manifested card happens to be a creature the opponent can afford to cast.

Resculpt

Targeting both artifacts and creatures makes Resculpt slig𝕴htly more flexible than the rest of the swap-removal staples, but the fact that it exiles is its chief benefit, since it denies cards like Emry, Lurker of the Loch from casting cheap and beneficial artifacts repeatedly.

Cyclonic Rift

Cyclonic Rift can be used to bounce a single permanent in case 🥃of emergency, but will traditionally be cast using its overload cost. It is also important to note that you can overload Cyclonic Rift as an instant to clear the ᩚᩚᩚᩚᩚᩚ⁤⁤⁤⁤ᩚ⁤⁤⁤⁤ᩚ⁤⁤⁤⁤ᩚ𒀱ᩚᩚᩚbattlefield on the end step of the turn before yours.

Whelming Wave

Blue itself has almost every Kraken, Leviathan, Octopus, and Serpent in the game, so you shouldn't worry too much when including Whelming Wave that it will miss most of an opponent's creatures.

Nevinyrral's Disk

Though it enters tapped, and thus requires some setup, Nevinyrral's Disk poses a problem for every other player the moment you untap it, because it becomes a one-mana board wipe at instant speed that is very hard to counter.

All Is Dust

All Is Dust might be the coolest card name in Magic: The Gathering history. It also happens to pair nicely with artifact-based decks that run lots of colorles💛s permanents, and blue has been synergistic with artifacts since 1993.

Rhystic Study

It's fair to say that Rhystic Study is not just an iconic blue card, but an iconic Commander card. The extra value it packs in Commander is a result of having three times as many opponents to trigger it.

Mystic Remora

In the same vein as Rhystic Study, Mystic Remora's value gets inflated quickly the more opponents you have. Unlike Rhystic Study, an early Mystic Remora stays a double-edged sword, since you won't have much left-over mana to cast those cards you're going to draw off of it.

Windfall

Windfall's benefits are three-fold: One, it discards any cards in your hand that you want to put in your graveyard. Two, it brings your hand to parity with the other players. Lastly, it disrupts your opponent's plans that they might have been trying to set up based on the cards previously in their hands.

Archmage Emeritus

Triggering Archmage Emeritus is very easy, especially in mono-blue where you're likely to be running a higher percentage of instants and sorceries than in other decks. This is especially useful when playing a control archetype, since your counterspells now replace themselves.

Brainstorm

As an instant, Brainstorm allows you to leave mana open to threaten a counterspell on your opponent's turns, then cast it on the end step of the opponent before you. It is also unique among cantrips in that it technically draws three cards, and you can place any two back.

Ponder

Ponder digs deeper than any other one-mana cantrip blue possesses, giving you a glimpse at🍃 the next two turns of card draw, and whether you want to shuffle what could be three duds away.

Gitaxian Probe

Gitaxian Probe gives you information about an oppone🐈nt alongside the other ben♊efits all cantrips possess for the price of two life, which is much cheaper than even one mana.

Mystical Tutor

Finding any instant or sorcery in your deck for a single blue mana means that on each opponent's turn, you can hold up Swan Song, and only tutor on the end step before your turn, giving you the card faster than casting it on your turn.

Solve the Equation

A less-powerful version of Mystical Tutor is still an appreciated piece of redundancy, especially since it pಌuts the🦹 card directly into your hand. This makes it harder to get rid of than leaving it on top of your library like its one-mana sibling.

Fabricate

With the same♛ up-sides and down-sides as Solve the Equation, Fabricate will be a boon to any mono-blue deck that relies on arꦜtifacts.

Midnight Clock

Not only does Midnight Cloc𒀰k act as ramp, but if it stays out for three turn cycles, you g𒊎et a free Timetwister that only affects you.

High Tide

While it doesn't bring the same degree of consistent ramp that a card like Cabal Coffers might, High Tide will almost double your mana production for a single turn. Mono-blue doesn't have to worry about having enough islands to make it mana-positive.

Sapphire Medallion

A special kind of ramp is cost-reduction, and no card works🔯 better for mono-blue than reducing every spell by one generic mana. The amount of mana saved by casting a Sapphire Medallion on your second turn will rival any c♈olorless ramp you could include in the deck, barring Sol Ring.