Sutra:
11. ivai mUnrilum vaiththuk koNdu periya thirumanthram prathAnam.

Meaning:
The Thiruvashtakshara manthra is considered as the first amongst the three vyApaka manthras that are higher than the avyApaka manthras.

Sri PBA Swami's Sarartha Deepikai:
The three important vyApaka manthras are:
1. Om Namo Narayanaya
2. Om Namo Bagavade Vasudevaya
3. Om Namo Vishnave
Amongst these three, the Ashtakshara manthra which is also known as the Periya Thirumanthra is considered the highest. Just as there is no being superior to Sriman Narayana, so also there is no manthra superior to the Ashtakshari manthra ("yadA sarveshu deveshu nAsti nArAyaNAt para: tada sarveshu mantreshu nAsti sAshtAksharAt para:"). Also, one can see the greatness of this manthra by the fact that it is the first to be taken in the Vishnu Gayathri manthra.