Opposite of ‘Irredeemable’ in Describing a Trait

I am helping a friend write a recommendation letter.

To me, he has one trait - tenacity - that makes him an excellent worker for his industry, as once he latches onto a problem, he persists in solving it, using all available resources.

I have a general sense of the word I want for that purpose: the opposite of irredeemable, as in, the one trait that makes him irredeemable is tenacity, but flipped in meaning.

Obviously, the direct opposite of irredeemable is redeemable, but instead, redeemable is more of a word to describe a trait that redeems a series of ‘negative’ traits, while I wish it to describe a singularly excellent trait, similar to how irredeemable refers to a singularly terrible trait. It also does not have the same dramatic effect of the word irredeemable.

One word I have come across that may be a good fit is ‘virtuous’, but it doesn’t nearly capture that aspect of excellence of a person due to one trait.

Any suggestions of a word that fits the bill? Specifically, a word that completes this sentence while giving a sense of excellence:

The one trait that makes him _____ is tenacity; he never gives up on a problem, and employs all his resources to solve it.

^{More than one word is also ok, as long as they’re still adjectives. Also, feel free to let me know if there is a better way to word my question and/or title. Thanks!}

Solution 1:

One word that fits well in that sentence would be invaluable.

"The one trait that makes him invaluable is his tenacity..."

Its literal meaning is that something is so good that it is impossible to place a value upon it.