What is the UML diagrams that should be considered as the most important to be mentioned in software technical documentation?

Class Diagrams, Sequence Diagrams, and Package Diagram.

The most important is the class diagram which expose the level of business understanding, but before it a more abstract diagrams help in understanding which are sequence, activity, and collaboration diagrams

Class diagram, workflow diagram and sequence diagram

class diagram is the best one

To my knowledge, UML can be classified as Structural and Behavioral Diagrams. Most of the Behavioral Diagrams can be provided by BA for understanding purpose. If it is not clear/not sufficient diagrams, then they can be done if required in Technical document along with the Structural Diagrams. 

Behavioral Diagram: State Machine Diagram, Communication Diagram, Use Case Diagram, Activity Diagram, Sequence Diagram, Interaction Overview Diagram, Timing Diagram

Structural Diagram: Class Diagram, Component Diagram, Deployment Diagram, Object Diagram, Package Diagram, Profile Diagram, Composite Structure Diagram

The importance of diagrams differs based on project and the diagrams can also be customized accordingly for the need. For example,  a technical document that is used in game development may consist Strategic Pattern, Chain of Responsibility, Composite Pattern and few more.