Elite Beat Agents, if you're into music games, is great. (I prefer Ouendan but you have to import that)
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Final Fantasy III is a great game, although old-school and challenging, but you probably prefer a challenge. Contact is fun, albeit a tad odd. (It's the same guys who did Earthbound, I think)...
I agree that Megaman ZX is a great game. Also, if you like touch-screen heavy platformers, Viewtiful Joe: Double Trouble is also pretty fun.