Most played cards in Mono Blue:

3 Decks
Delver of Secrets
12 copies - 4 average
Daze
12 copies - 4 average
Brainstorm
12 copies - 4 average
Wasteland
12 copies - 4 average
Force of Will
12 copies - 4 average
Ponder
12 copies - 4 average
Gut Shot
10 copies - 3.33 average
True-Name Nemesis
10 copies - 3.33 average
Stormwing Entity
9 copies - 3 average
Ethereal Forager
8 copies - 2.67 average
Preordain
8 copies - 2.67 average
Misty Rainforest
7 copies - 2.33 average
Polluted Delta
7 copies - 2.33 average
Scalding Tarn
6 copies - 2 average
Grafdigger's Cage
6 copies - 2 average
Force of Negation
5 copies - 1.67 average
Brazen Borrower
5 copies - 1.67 average
Snow-Covered Island
4 copies - 1.33 average
Flooded Strand
4 copies - 1.33 average
Swords to Plowshares
4 copies - 1.33 average