What are the must have skills of an excellent trainer?

Excellent trainer must deliver his/her knowledge the best way you to remember and to stay focus on it.
It will be by practical or theoretical, or both, the aim is you to understand the material and to be able to use it and to share with others.
Other must have skill is sense of humor, communication, adaptable to flexible questions/answers lecture according to the needs of the participants.

The most vital skill, to express his knowledge in a professional way.

A trainer should be able to asses the level of skills, language ability, topic knowledge already acquired by the attendees to a course and adapt the course direction and content accordingly.
There could also be several differing types of instruction ie reading, listening writing and interaction between all participants as knowledge and skills can be learnt in a variety of manners and not every one learns in the same manner.

being a leader not only a boss

the most important skill is how to make the audience get the Knowledge and use it.
he need to talk to all types of people using the word, sound and pictures, and let the audience do it him self (the trainer to be facilitator).

Considering the age of participants*; If we take the example on adult learning we should mention "Andragogy Principles" and training techniques*; That should be vary to ensure all participants collaborations.