This is the 2764th most frequent Russian word.
лучшая
"Best" (feminine form).
In this sentence, 'лучшая' is used as an adjective meaning 'best', modifying the noun 'подруга' (friend) and showing a superlative quality.
Она была моя лучшая подруга.
She was my best friend.
Here, 'лучшая' serves as an adjective meaning 'best', describing 'возможность' (opportunity) as superior in quality or nature.
У нас есть лучшая возможность сделать это.
We have the best opportunity to do this.
In this context, 'лучшее' is the neuter singular form of the adjective meaning 'best', agreeing with 'решение' (solution), which is a neuter noun.