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Its not just some random thing Rick came up with, since he also apperantly knows to not make Morty too cocky about himself. It has to mean something. If it means that they are the most "themselves" then this implies that our Rick could be the smartest and realest of them all. Following this pattern Morty could have the biggest potential for growth of them all. That our Morty is the most likely to surpass Rick. The previous Mortiest Morty Rick had could be further evidence of that.

Evil Morty could be the previous Mortiest Morty and it shows that Rick used his potential for growth, to the point where he surpassed him before he even went into adulthood. leading to him abandoning that Morty. That would be actually sick if Evil Morty was actually the only candidate in the entire universe to be smarter than the smartest being in the universe But it could also mean something completly else. who knows.