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counterspell, it's cheap and gets the job done, they are even making a
"BB(1)
counter target spell"
in Time Spiral, and THAT is a decent counterspell so ^^

yah probably, don't remember the name right now, and I don't feel the need of checking it put n_n

Voidslime does nothing against the split second ability...

Well, Split second owns spells and activated abilites. So yeah, something like decree of silence would prevent split second.

It'll rape in metagame.

I say Last Word since it says NO and it works. The only way to stop a Last Word is Time Stop / Spelljack and I doubt the average opponent will be packing those answers.

Aside from the namesake spell "Counterspell"

My favorite counterspell spell would be Arcan Denial, sure it let's your opponent draw two extra cards after that turn. but it also let's you draw one card for you as well (Though if you counter a spell with AD during your turn, your opponent will be doing a whooping three-card-draw from his library [2 on upkeep, 1 on draw phase]).

For Library-killer decks and or deck's which can control the amount of spell played per turn or general counter-decks, Arcan Denial would be a good addition. Seeing that it costs 1B, it could also work for multi-colored decks. Arcane Denial is best played during the first few turns or when you can counter more than one spell at a time.

mana drain.