Which form of marketing is best: SEO, PPC or Digital Marketing?

SEO and PPC are a part of Digital marketing. SEO, PPC, Display media and content marketing are the tactics used to achieve goals like productivity, etc. You can't do PPC or SEO without a content marketing strategy. Scores of factors go into this, the main point is how to use a strategy to drive a traffic immediately.

SEO and PPC are types of digital marketing of which search engine optimization is better as it helps to rank keywords higher in search engine pages to generate good traffic and leads for your online business.

I perfer SEO industry because i'm in it.
But its yours choice.

SEO is still the most desirable service and it has the highest chance of leading to conversion.