It would be more typical to use "a: indefinite article" in your example sentence: The most beautiful love is a difficult love.
We usually "a" - indefinite article - to refer to something nonspecific. In this case, there is no apparent specific love to which a speaker of this sentence would refer. However, using "the" - definite article - is correct.
It is very difficult for some students to learn how to use articles correctly, and this sentence - an exception to common practice - does not help matters. However, I hope my reply helps you.