Data is still being gathered on what cards are the best. For now, the best abilities are the Sonic Raid/Ars Arcanum/Break abilities that provide 6-8 hits on a single target, and Mickey that gives a single large damage to all targets.
Some medals may be great at 5* but overtaken by others at 6*, some may be mediocre at 5* but awesome at 6*.
For now, most of the KHII Sora and Rikus, Mickey (Magic), Hercules (Power), Cloud (Power), Stitch (Speed) are arguably the best ones.
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It is hard to provide a good JP site now. We don't have main sites like gamefaqs where you can go straight to for information on new games. JP guides are mostly wikis and message boards, and there are dozens of them. You have to wait on which gains the most views over a long time and the small ones to die away to know the best sites.