I am looking for a word to describe a person who is good at his job and therefore everybody needs him.
Is it 'needed' or 'needful'? What about 'indispensable'?
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Indispensable is a good choice.
His knowledge of human nature, inexhaustible energy, dauntless self-confidence and diplomatic finesse made him indispensable to Gustavus III. Source
Also, essential, crucial, vital, or key.
She is a crucial member of our team and would be difficult to replace.
None of these words are specific to a person who is good at their work - they can be used for anything that is very important. I've chosen the ones that I see most often in business communications - there are other synonyms for indispensable that would work as well.
We could probably narrow it down even more if you provided an example sentence to show how you would like to use this word. I asked my husband what words he would choose, and he said "under-paid", "over-qualified", and possibly "over-worked" (as a joke).