This is the 289th most frequent Russian word.
самым
The word "самым" is the instrumental singular form of "самый," meaning "the most" or "the very" (used for emphasis).
In this sentence, 'самым' is used as an instrumental case of the superlative of the adjective 'разумный,' emphasizing the described attribute.
Он достиг успеха самым разумным способом.
He achieved success in the most reasonable way.
In this context, 'самым' is employed in the instrumental case as part of the predicate nominative to denote the supreme importance of health to the subject.
Самым важным для нее было здоровье.
The most important thing for her was health.
Here, 'самый' is used as the superlative form of the adjective 'красивый,' describing the city's beauty as unparalleled.