By far the ephors. Most of the nonsense that gets spewed about how the Lacedaemonian constitution had a system of checks and balances comes from Plutarch. Thing is, Plutarch is writing hundreds of years after the Spartans had already colored their own history with the "Spartan Mirage," and what's more his agenda is to show the various states of Greece as displaying a particular virtue. Sparta's virtue, for Plutarch, is morality and discipline, and most of what he says is colored by this. Our most accurate source is by far Aristotle. Many people like to claim Xenophon as a good source, but while he's quite knowledgeable about the army his access to the core of Spartan society was severely limited and what he says about it doesn't always make sense. Aristotle makes it quite clear that the kings have no more than ritual and ceremonial power, rather like the Archon Basileus did at Athens. The ephors could try kings for crimes, have them exiled, execute them, depose them, you name it. The kings had no such powers. What's more the five ephors really didn't answer to anyone. While constitutionally there were a series of assemblies and courts which theoretically had control over the ephors, in reality the ephors controlled everything. Since the ephors had the power to disenfranchise any Spartiate and had the power to sentence any non-Spartiate without trial they essentially could deny all political power to everyone who wasn't an ephor. Thucydides is full of instances where the ephors completely abused their power and overrode the already weak-willed assemblies to get their way