This is the 2308th most frequent Russian word.
лучшим
"Лучшим" means "the best" in English.
лучший is used here as an adjective meaning 'best', describing the quality of the subject 'день' (day).
Это был его лучший день.
It was his best day.
лучший is used in its comparative form, indicating an effort toward improvement or a higher quality (abstract concept).
Мы стремимся к лучшему.
We strive for the better.
лучший is used as an adjective describing the noun 'друг' (friend), signifying the highest degree of quality in a friendly relationship.