Serra's Emissary

Protection from a card type is the widest defensive keyword the game offers, and this hands it out to your whole board on a single choice as the Angel resolves. Naming "creature" shuts off an entire attacking army at once: those creatures cannot block your board, cannot get through unblocked, cannot deal combat damage to your creatures. Naming "instant" or "sorcery" blanks the burn and removal that would otherwise answer a seven-mana threat. Most protection-granting designs cover one permanent for a single turn, or shield from a color rather than a whole card type; here the umbrella extends over every creature you control and holds as long as the Angel lives. Protection cuts both ways, though, and reading the room means reading your own list too. Name "instant" and your own instants can no longer target your creatures; name "enchantment" or "artifact" and your Auras and Equipment slide off them the moment the choice locks in. That constraint is why "creature" is so often the pick: it warps the opposing board without touching your own toolbox. The 7/7 flying body is unremarkable for seven mana; the choice made as it enters does all the work, converting a fair evasive beater into a soft lock. The answer is almost always "kill the flier first," and a board built to attack profitably into it frequently cannot. It reads like a finisher and plays like a prison.