10. Pack Rat - 44
No longer just the bane of Return to Ravnica block limited, Pack Rat stomped onto the scene at Pro Tour Dublin - though Mono-Blue ultimately stole the spotlight. While it was initially run as a 2-of in most lists, it didn't take long for everyone to adopt its 3rd and 4th brothers. It might be weak to a few mass removal spells, but against spot removal heavy lists you can simply hit 3 mana and start to overtax that removal by making every draw another rat. Eventually your tide of rats will simply overwhelm them. Not even the introduction of Bile Blight has stemmed the numbers. It's also a card that's benefited from having a few built in synergies. Underworld Connections if nothing else draws you an extra Pack Rat each turn, and Mutavault, even just animating itself, will suddenly Glorious Anthem your team of rats. The damage jump from the first few turns of Pack Rat activations with a Mutavault in play is impressive and surprising if someone isn't carefully doing the math.
9. Detention Sphere - 44
Oblivion Ring, but with an upside, was no doubt going to see some play. Even putting aside its ability to generate advantage when your opponent plays out more than one target, Detention Sphere has been useful for its catch-all properties. Enchantment, Planeswalker, Creature, etc. it doesn't matter; Detention Sphere is here to put it in time out. It's been an invaluable tool in Blue & White based control decks, and is likely making this list based on its increasing play in other decks as well. One of the new directions for Mono-Blue has been to splash into White for Detention Sphere, Ephara, God of the Polis and some choice sideboard tech. If you have some room in your 75, some enchantment removal isn't the worst inclusion, since you're inevitably going to be facing down this card.
8. Desecration Demon - 45
Desecration Demon might be worthy of some kind of award for best reversal ever, at least in recent memory. Punisher-style cards have always been a bit contentious, but the dream of an evasive 6/6 for 4 was basically dashed upon a sea of Lingering Souls tokens and undying creatures for the first half of Desecration Demon's life in Standard. I personally remember chuckling every time I saw one resolved; knowing the Naya Blitz deck I was on at the time was going to run them over the next turn. While the big demon can still be a little awkward against some fast draws, these days though, that kind of hyper aggro is in remission. Getting to play an under-costed creature is a greater reality when the format is as slow as it is.