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Going off of demonstrated abilities, Alien X/Celestialsapien should win, though a simple Naljian children's toy was able to trivially replicate the Omnitrix and the Omnitrix can scan and copy the DNA of Celestialsapiens, so the Naljians might be able to beat the Celestialsapiens in a war. We don't know, though, as we've only seen one Naljian and it only appeared in one episode.