Apart from numerous other significances of 'saum' thr most significant feature of 'saum' is that the Prophet Muhammad - Peace be upon him - said; " what ever a person does (good deeds) he is rewarded from 10 times to 700 times, but the reward of 'saum' is exempted from it. And the Allaah says; 'as this 'saum' is for Me, only I shall reward at My Own (to the person who observed 'saum')'. [Bukhari, hadith 1894, Muslim - hadith 2707]
Other features of 'saum' are;
The smell of the mouth of saum-observing person is better than 'musk'. [Bukhari - hadith 1894]
There is a door of 'janna' (paradise) called 'Rayyaan' and only a saum-observing person can enter into 'janna' through this door. [Muslim - hadith 2710]
However other 'ibada' have their own significances. The highest significance of 'sala' (prayers) is that it was gifted by Allaah Himself to Prophet Muhammad - Peace be upon him - during 'Mairaj' and above the seven heavens.