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I'm looking for a single-word adjective that means either "the best at what you do" or "the most experienced at what you do." If forced to pick one of those two perspectives, I would choose the former. I will be using it in the following list:

• Cadet


• Junior


• Senior


• Veteran


• This is the word I'm looking for

Example sentence: John is a ______ mechanic.

The word does not require a military context, despite my above list having a military feel to it.

"a master" mechanic or "the consummate" mechanic would fit

primo, TFD perhaps an elaboration of prime.

slang: exceptionally good of its kind; first-class, highly or most valuable.

As in:

John is a primo mechanic.

and Vocabulary.com

She completed her chief residency and surgical retina fellowship at
one her field’s primo facilities there. Washington Post Sep 9, 2018

Generally used for artists, a good candidate is:

Virtuoso.

I'm recommending this word because I feel that when people get really good at something, they do that thing as an art - as if playing with/in it.

Meaning:

• a person highly skilled in music or another artistic pursuit.

Example usage: a celebrated clarinet virtuoso | [as modifier]: virtuoso guitar playin.

_{I looked up the exact meaning in Apple's dictionary. Feel free to look it up anywhere else for more details.}

The French 'sans pareil' (without a match, better than any other) also exists in English in the form of 'nonpareil' used after the noun, pronounced 'non per rail', more or less. It is a literary term says the Cambridge Dictionary Online. Their example sentence is

He was a teacher nonpareil.

Your sentence, then

He was a mechanic nonpareil.