bonus

Take the rank with a pinch of salt. bonus sits at the tail of this list because of what our particular corpus happens to contain, not because it is a rare or advanced word: it is one of the first adjectives any Latin course teaches, and on most frequency lists it would be somewhere near the top. Treat it as a core word that landed here by accident of counting.

It is a first-and-second declension adjective and entirely regular in the positive: bonus, boni, bono, bonum, bono in the masculine, bona, bonae, bonae, bonam, bona in the feminine, bonum and bona in the neuter. The irregularity is all in the comparison, and it is suppletive, meaning the other degrees come from entirely different roots: comparative melior, melius, "better"; superlative optimus, optima, optimum, "best". The matching adverbs are bene, melius, optime. There is nothing to derive here, so learn bonus, melior, optimus as a chant, the way you learn "good, better, best".

The neuter works as a noun: bonum is "a good thing, a benefit", and the plural bona means "goods, property, an estate". Phrases you will meet constantly: bona fide, in good faith, which English took whole; vir bonus, the Roman shorthand for a decent man; summum bonum, the highest good, the target of every ancient ethical system. Descendants: Italian buono, Spanish bueno, French bon, with melior surviving intact as migliore, mejor and meilleur.