I think Ultron would be the ultimate choice here for me.
Can you imagine having him on the loose? He is the ultimate Supervillian with superhuman levels of strength, speed, stamina, durability, and reflexes. He would be pretty much unstoppable in our world unless The Avengers existed to. Who knows even if they could stop him in the end? Scary thought.
By the way, in case you are wondering what this card is, it's a 2015 Upper Deck Marvel Dossier #12 ULTRON Foil Parallel. Despite how scary Ultron would be in real life, the card of him isn't. It's actually pretty sharp with the gold/rainbow shine.
What character on a card would you choose that you would find scary if existed? Let me know in comments and tell/show me which card it is and why you chose it.