When I started asking pro players what their most feared card in Sealed Deck was, there was a common answer: Archangel Avacyn. It turns out the legendary angel is quite strong. I then pressed on the pros to let me know the next card on their list:
Four-time GP Top 8 competitor Chapman Sim: The planeswalkers and Drogskol Cavalry. Once any of them get going, it's pretty difficult to stop them
Grand Prix Taipei Champion Huang Hao-Shan: I'm pretty afraid of Sorin, Grim Nemesis, but I'm equally afraid of Red-Green Werewolf decks.
Four-time PT Top 8 competitor Lee Shi Tian: Ongoing Investigation. When it goes online you just can't stop the card advantage. You trade and then they exile their creatures and just continue getting Clues.
Pro Tour Hall of Famer Kenji Tsumura: Ever After. It's really good in long games and in this format the games are either very long or very fast.