I guess the most frequently used kanji (かたい）is 　固い　（⇔　緩い　（ゆるい））.　From these two, I would say 硬い, but depends on the sentence, we change the kanji. So it's hard to say which one is the most.... Btw, 硬い　⇔　軟らかい　、　堅い　⇔　もろい　this idea helps me to figure out which one to use. Hopefully, this was helpful.

It's antonym. 「固い」⇔ loose 「硬い」⇔ soft 「堅い」⇔ fragile (or weak)

硬い」と思う。