Milling became so popular that it's now common to see entire decks built around the keyword

The Thing 39%inclusion 309 decks 789 decks The Astonishing Ant-Man 7.9%inclusion 212 decks 2.68K decks Bruce Banner 1.2%inclusion 59 decks 5.12K decks Captain Kirk, Boldly Going 33%inclusion 13 decks 39 decks Crystal, Inhuman Princess 0.6%inclusion 13 decks 2.34K decks Top Commanders The Wise Mothman 75%inclusion 24.2K decks 32.2K decks Atraxa, Praetors' Voice 10%inclusion 4.15K decks 42.9K decks Tidus, Yuna's Guardian 19%inclusion 3.61K decks 19K decks Hakbal of the Surging Soul 15%inclusion 3.60K decks 24.4K decks Xavier Sal, Infested Captain 26%inclusion 2K decks 7.83K decks Zaxara, the Exemplary 7.9%inclusion 1.52K decks 19.3K decks Magus Lucea Kane 10%inclusion 1.49K decks 14.7K decks Ezuri, Claw of Progress 20%inclusion 1.44K decks 7.33K decks Esika, God of the Tree 5.4%inclusion 1.33K decks 24.9K decks Yuna, Grand Summoner 12%inclusion 1.27K decks 10.2K decks Animar, Soul of Elements 5.6%inclusion 1.26K decks 22.7K decks Leonardo, the Balance // Michelangelo, the Heart 15%inclusion 1.24K decks 8.19K decks Zimone, Paradox Sculptor 31%inclusion 1.19K decks 3.85K decks Owen Grady, Raptor Trainer // Blue, Loyal Raptor 19%inclusion 1.03K decks 5.41K decks Sab-Sunen, Luxa Embodied 26%inclusion 1.01K decks 3.83K decks Ms

In addition to some serious card draw the graveyard hate that is stapled onto it is useful in its own right
So if you have Swamp out and Reflecting Pool, your Pool only makes black mana
Manriki-Gusari is a real honorable mention here, since this was an equipment from Saviors of Kamigawa that was exclusively played to be able to destroy opposing Jittes
Haunted Library, Occult Epiphany, and Priest of the Blessed Graf put out tons of tokens and Storm of Souls fills the board with everything that's died thus far