The best of everything.
1st: Cumberbatch at the height of his powers giving us a seamlessly brilliant performance - physically, intellectually and emotionally. Not to mention comically. He's never looked better, been more perfectly cast, or given us a subtler or more believable character arc..
2nd: The special effects. The world-bending is impressive, of course. But the "simple" effects of astral bodies and mandalas of light immerse us in a reality that becomes our own. The Cloak of Invisibility deserves it's own Oscar.
3rd: The story. This is the origin story that truly captures the sense and tone of Marvel, that set it apart, always, from all other comic purveyors. Doctor Strange was always the most unique, because nothing came from outside. He was not bitten or radiated or alien or a mutant. He's a man, just a person, and his power was always inside him. He'd already chosen the path of serving and saving. Now, he only has to overcome ego to actualize his true power.
4th: The Cast. It's a wonderful thing to be surrounded by this deepest bench of amazing talent. Every line, movement, interaction rings true to the world of the movie and connects the audience to the characters.
It just doesn't get better than this.