Frogger on 14 July 2015
I'd start with Platinum. It's the oldest Pokemon game that still retains the most unchanged battle system. It's also the best. Great, epic story. Good world building. Good Pokemon. Fun and interesting characters. A ton of content throughout the region both during and post game. Most interesting region geographically by far, second only to HGSS Johto.
Platinum will make you fall in love with Pokemon. No better place to start.
I would advise against XY though. It lacks all the charm of the older games and the story is terrible and the characters are terrible. I feels like you're playing an episode of Barney, which is not the impression anyone playing Pokemon for the first time should get about the entire series.
