The strongest contribution of Pallavas seems to be their commitment in making Kanchi a university town (in the modern sense of the word). Reading Chinese chronicles of the 5th and 6th centuries (Xuanzhang), one is impressed with the respect shown to Kanchi on par with the other great university town of India (Nalanda). However, Kanchi was different in that it was not a predominantly Buddhist town, it had the great Sanskrit schools of Baravi etc as well as the Tamil schools run by Saivite scholars.
My contention is that to build such a town, one needs deliberate imperial policy to support a environment that encourages such a wealth of learning. This is something that is missing from the later Chola empire. The Chola capital was never a centre for learning and there was no effort to encourage/make the Chola heartland a centre and hub of international learning.
Even Harsha's Kanyaghubja and Salukkya Vadabi were never on the same league as Kanchi! More importantly, as I have noted before, Chinese pilgrims usually had an inflated view of their own civilisation - but even they too came to Kanchi to learn.
This brings me to one point about the use of Sanskrit/Prakrit in Kanchi. This has been used to say that the Pallavas had no love for Tamil. But think about it, if you had a town that was known for both learning and commerce, where people from many different nations converged - what would you do? You would encourage a lingua franca that all could speak. This could be one reason why Pallava inscriptions were rarely in Tamil - they were meant to be understood by people from many different parts of the world.
Nagaresu Kanchi!
id 300s to 690 or so, there was a Hindu Kingdom called Tarmanagara (Dharma nagaram) in Java, near where Jakarta is. Its most famous king was Puranavarman. In fact all its kings had the name varman. And they used the same script used in Pallava inscriptions. Purnavarman was the first Javanese king to leave behind an inscription - and this inscription still exists! In it, Purnavarman writes in Sanskrit that 'this is the inscription of Purnavarman who is like Vishnu'.
In fact, the earliest Hindu artefact found in Java is a Ganapathy statue, found on a place called Mt. Raksha. (Kalki's Paranjothi would have been glad).
There are many other reasons for praising the Pallava contribution, but this policy of educational enlightenment regardless of which religion the knowledge came from must be praised.