Wala namang best or not best. It depends on your niche yan or yung specialization mo. Like in my case I'm into web development niche at may kaunting graphic design and bookkeeping. It could be anything na gusto mo basta you're good at it para ma hire ka.
Nag start ako as a VA more than 10 years ago na walang formal training. Those days hindi pa sikat ang term na VA. More of an online worker ang tawag samin. This time around madali nalang makahanap ng traning dahil nagkalat na ang mga libreng resources via Google and YøùTùbé.
Kapakanan muna ni client before yours. Pwede ka kasi ma blacklisted kung pumalpak ang project due to inexperience. If confident enough ka naman na magagampanan mo ang magiging role mo then go ahead and grab the opportunity. If not, then tread wisely.