How do I make a video game boss memorable?

Make a memorable transition from the level or dungeon to the boss. For example, if your boss is early in the game, give it an intimidating intro and a satisfying death. If your boss appears later in the game, such as near the end or as an important character, make them do something that the player will absolutely despise them for, such as causing some traumatizing recollection of events in your main character, or permanently killing off a side character. Alongside buildup, don't forget to add great music and atmosphere too. Basically, to make a boss feel memorable, you have to make the player relate to what the character(s) in the game is/are feeling as best you can.