[jonahbenton]

Friend, since meditation is usually a solo, silent activity, the best literal answer to the question is the "self."

Others may meditate or work in therapy/reflective contexts with other humans.

The notion of "self" is fuzzy as there likely are numerous physical and logical biological components that participate in self-talk and response and we (I) simply don't know enough physical and logical brain architecture.

I don't reject out of hand the notion of capital-S "Self" as "God" but "God" is essentially trademarked by multiple distinct authoritarian power structures around the world and their presence, policies, and activities I generally though not in all cases despise. I don't find value myself in promoting that association.

Cheers.