Speaking as one of the assholes who plays unfair decks (

), the cards that seem to piss people off the most are the ones that are good exclusively in duels rather than multiplayer. I.e., land destruction, countermagic, Rofellos as a general, anything that has ever been banned/restricted (Yawgmoth's Will, Tutors, Tolarian Academy, etc.) and other cards that were at some point the basis for Tier 1 tournament decks (Survival/Recurring Nightmare, etc.)