What is the ??? if not the ultimate value of an act?
Good question. I can’t say for certain what the Rambam meant by ??? but if it’s just another value (albeit the best value) then there is no reason why it should be considered “better” to pursue this value over any other value.
Working with your argument, that Emes is a motivation because of its value – surely one can distinguish between two values. If a hungry person has a choice of eating a steak or leaving it for somebody else and chooses to leave it, simply because he wants to be nice to the other person, you would call him a better person. And the value of eating the steak is more than the value of leaving it, yet the decision to prioritize the ‘noble’ value over the more-valuable selfish one is Good. Would you not agree with that?