In my perspective, anchored in a neurobiological understanding of the world, the 'simplest' explanation would be the one that aligns most closely with what we know about how the brain works. Ontological simplicity and explanatory simplicity aren't always at odds. A theory that posits fewer entities, if those entities are consistent with neurobiological data, may provide a more unified explanation for a range of phenomena.
Determining 'simplicity' isn't an arbitrary decision made by a single person. It's a collective process in the scientific and philosophical community, grounded in evidence. The 'simplest' explanation will be the one that best fits the data, explains the most phenomena, and makes the fewest unsupported assumptions.
Remember, Occam's Razor is a heuristic, not an absolute rule. Simplicity is one among many factors in theory choice. Others include empirical accuracy, coherence with other accepted theories, and explanatory breadth and depth. So, simplicity is a guide, but it doesn't always dictate the best theory.