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Maybe Serena next since I use her so frequently and have her at 4/5. But she's a Tech character and I don't really have much use in using her sync since she doesn't even Mega Evolve or change form. She's mostly for trapping, viral Hypnosis, and stat draining rather than damage output.
Just finished maxing out all the gear in the game too (1* and 2* of every type at level 15) and it took a good 10 million coins of investment over a year and a half.