Not all languages have adjectives; some use adjectival nouns ("red.one" instead of "red") and/or stative verbs ("be.red" instead of "red").
Among languages that have adjectives, not all allow comparison of adjectives, and not all adjectives take agreement suffixes.
What I haven't been able to find on the Net is which kind of inflections for adjectives are the most common across languages that have a word class of adjectives that is morphologically distinct from, say, genitives and relative clauses. No inflections? Inflection for comparison? Agreement? Something else?