I've been in search of the best Spell Queller deck in Modern ever since the card came into existence. I believe it is the most powerful card for Modern in Eldritch Moon and it fits my playstyle perfectly. People think of me as a "white weenie guy" but if you look at the decks I play, they tend to be highly interactive. My favorite creatures are ones that can beat down and disrupt the opponent. Spell Queller does exactly that!
One of my favorite archetypes in Modern is Hatebears. There is currently G/W, B/W, and mono-white versions of Hatebears. With the addition of Spell Queller into Modern, I believe U/W Hatebears is now viable!
The basis of any hatebears or Death & Taxes strategy is disruption and pressure. Mana denial is the primary focus, which, in Modern, is accomplished via the combination of Ghost Quarter and Leonin Arbiter alongside Thalia, Guardian of Thraben. Mausoleum Wanderer and Spell Queller play perfectly into this strategy since they can apply pressure while countering the one spell the opponent can cast on their turn.
Reflector Mage is also great because creatures are about the only spell type that will resolve through Thalia, Guardian of Thraben and Mausoleum Wanderer, and Reflector Mage helps take some of the load off of Path to Exile while providing an incidental body to continue the pressure. It's also quite the combo with Restoration Angel.
The deck feels pretty powerful and gets a lot of free wins from the mana denial plan, especially in a world dominated by fetch lands, which is exactly what Modern is.
What to Sideboard Out and When
Against decks that are light on creatures, I board out some copies of Path to Exile and/or Reflector Mage.
Against decks that don't have many non-creature spells that cost three mana or more, I'll board out some or all Mausoleum Wanders.
Against decks running Electrolyze, I'll shave some Flickerwisps.
When Stony Silence comes in, AEther Vial comes out.
I'll board out some number of Thalia, Guardian of Thraben against decks that are spell-light, especially on the draw.
Spellskite and Phantasmal Image are the flex slots in the deck and can be boarded out liberally.
I pretty much never board out any copies of Restoration Angel, Spell Queller, or Leonin Arbiter.
What to Sideboard In and When
Sunlance comes in against Affinity, Wild Nacatl, Goblin Guide, Infect, Merfolk, Elves — pretty much any deck with a reasonable number of targets for it.
Stony Silence comes in against Affinity, Tron, Lantern Control, and any sort of artifact-based deck. I'll only bring in Stony Silence when it's an absolute blowout against the opponent, as it is in the specific matchups mentioned, because AEther Vial is so important to the deck.
Rest in Peace can be just as much of a blowout as Stony Silence against certain decks. I bring it in against Living End, Dredge, Storm, and any graveyard-based combo deck. Unlike Stony Silence, I'll bring in Rest in Peace as just a value card too; against decks running Tarmogoyf or Snapcaster Mage I'll bring in one to two copies since the only card in the deck effected by Rest in Peace is Moorland Haunt, which only plays a minor role in our game plan.
Selfless Spirit comes in against Supreme Verdict, Anger of the Gods, Pyroclasm, Whipflare, Oblivion Stone, and any time we're likely to engage in combat against similarly-sized creatures.
Meddling Mage comes in against combo decks such as Ad Nauseum, Scapeshift, Living End, and any deck reliant on a specific card.
Selfless Spirit and Meddling Mage are the most versatile sideboard cards in the deck since they can attack, so I bring those in any time I have an Abundance of cards to board out, like against creatureless decks where I want to 'board out all four Reflector Mages, all four Path to Exile and just need to replace those cards with cards that impact the matchup in some small way.
Overall I'm very happy with this deck and think it might just be the best home for Spell Queller in Modern!