Most played cards in Mono Blue:

3 Decks
Sea Gate Restoration
4 copies - 1.33 average
Arcanist's Owl
4 copies - 1.33 average
Nyx Lotus
4 copies - 1.33 average
Omen of the Sea
4 copies - 1.33 average
Opt
4 copies - 1.33 average
Mystic Subdual
4 copies - 1.33 average
Thassa's Oracle
4 copies - 1.33 average
Corridor Monitor
4 copies - 1.33 average
Gadwick, the Wizened
4 copies - 1.33 average
Brazen Borrower
3 copies - 1 average
Barrin, Tolarian Archmage
3 copies - 1 average
Sublime Epiphany
3 copies - 1 average
Stern Dismissal
2 copies - 0.67 average
Shark Typhoon
2 copies - 0.67 average
Mystical Dispute
2 copies - 0.67 average
Thassa, Deep-Dwelling
2 copies - 0.67 average
Glasspool Mimic
2 copies - 0.67 average
Negate
1 copies - 0.33 average