I know "intelligent" is a subjective description, but between my two cats Boycat is definitely more intelligent than his sister! So many examples, but a few are:
He has cracked any puzzle feeder I've tried in minutes. Girlcat stares at them blankly and walks off.
Unlike Girlcat who is surprised whenever I put her in the carrier to go to the vet, Boycat knows it's coming. He will also learn from the previous times and try different strategies to evade capture.
Boycat is also much better at communicating his needs, for example if he wants to be played with he will walk to his toy box and get your attention. Once when I couldn't feed him breakfast due to a vet procedure, he went and got his treat toy from another room and batted it around in front of me while yelling and making a huge racket (never done this before so he was definitely trying to tell me to feed him!).
How intelligent (or not so intelligent) are your cats?