Serra's Emissary is an awesome Mythic Angel from Modern Horizons 2.
So what card types are we most likely to choose? The first obvious choice would be creatures. Suddenly your creatures are all unblockable and you are completely immune to combat damage! Seems pretty powerful for a 7-mana 7/7 flier!
In matchups where your opponents aren't running creatures, the next most likely choice seems like it will be instants. Serra's Emissary protecting itself and all of your other creatures from cards like Path to Exile can certainly be game ending.