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If you're new to the challenge, start with either Yellow or Alpha Sapphire. The former is really not difficult, just dated by its mechanics but the AI is incredibly stupid, while Alpha Sapphire is at least fairly forgiving. You can also save your Route 119 encounter for a 100% Feebas fishing spot, as I highly recommend Milotic.
If you're experienced, White 2 will be right up your alley.