Is cilantro the best substitute for epazote? How similar are their flavours ?

I recommend looking for epazote in dried form. Try looking at a hispanic market--they'll be sure to have it. I live in the middle of nowhere, TN, and I can find it without too much fuss. It's also called herba sancti maria or wormseed.

Epazote has a fragrant and flavor that's closer to oregano. I would try Mexican oregano as a sub.

They are two different herbs. You could use both in combination (or individually) but neither is really a substitute for the other.