Is petroleum ether the best chemical to extract useful content from medicinal plants?

There is no such thing as "the best" solvent. It all depends on the nature of the chemical you wish to solvate/extract.

For example, the polar, water soluble Vitamin C cannot be extracted by petrolium ether because of the non-polar nature of the latter.

Nonetheless, I think petrolium ether is a pretty good solvent for non-polar chemicals. We've used it successfully in extraction but we've used other solvents successfully as well.