Darksteel Mutation Exxaxl: This might seem like a weird pick to some, but completely strip your best creature of anything it can do can be an incredibly impactful play on an opponent's board state
For example, I use a lot of Dragon Shield sleeves, and they all come in a box which can't hold a 100 card sleeved deck, but the box can be reused to hold extra sleeves (you know the extra few that come in each box or some that didn't come in a box at all), or your favorite/most used tokens, or maybe even (part of) that stack of cards sitting on your desk right now
It wont hit creatures, but if the primary threat is counterspells, Duress does the job
Eldrazi decks and colorless decks get a free second copy of Cyclonic Rift, if they want it, thats castable for free when theyre ahead
Staff of Compleation draws you cards and has plenty of other effects