It's "una excusa mejor". In Spanish, the only difference between comparatives and superlatives is the position and the presence of the defined article in the superlative. So "una excusa mejor" is "a better excuse", while "la mejor excusa" is "the best excuse".
(Actually "una mejor excusa" would be correct for "a better excuse", but it sounds old.)
PS: Wouldn't this topic go in the grammar section, rather?
No, the Vocabulary forum is for the translation of words, phrases and idioms -- in other words, it's for questions about meaning. Questions about word order, sentence structure, parts of speech and other questions should go in the Grammar forum. I've moved this thread to Grammar.