What are the most important things to include in your thesis defense presentation?
Tailor the presentation to those who assess it. You say that will be your supervisors. If that means they are people closely familiar with your subject matter, you can devote more time to project details. That means 60-70% of your time. If they are non-experts (researchers from a different department/specialization), spend less time on technical details. When in doubt, always focus more on 

-What's the big question/motivation for this study

-how does your contribution tie into big question

-Why was it a difficult project to tackle (important for non-experts)

-how did you overcome those difficulties

-how can the big question be addressed now that you made your contribution?

The last ties back into the beginning and may feel repetitive to you. But it is important for people listening to the talk.