As someone who was throwing the most popular parties in my hometown¹ during my youth probably the number 1 thing is finding the right mixture of people.

Nearly the most boring thing you can do is only inviting people who know each other, ideally it is an explosive mix of different ages, backgrounds, interests, styles to avoid people sticking together in their known constellations.

¹: one if the proudest moments was when some random stranger in an European capital spoke to me on the street and told me: "Hey I know you, I have been to your party!" and I had no idea who they were