Perfection in showing gratitude to someone is saying "Jazāk Allāhu Khayran"
the Messenger of Allah ﷺ said, "Whoever some good was done to him, and he says: 'May Allah reward you in goodness' then he has done the most that he can of praise."
قَالَ رَسُولُ اللَّهِ ﷺ «مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَقَالَ لِفَاعِلِهِ جَزَاكَ اللَّهُ خَيْرًا فَقَدْ أَبْلَغَ فِي الثَّنَاءِ۔»