Depends on what you're gonna do, which is hard to know before you start. If you're just playing one, I'd go with Black or White. If you're thinking of playing multiple, I'd start with one of the older ones....or better, the remakes.
That, that, that
If you aren't sure about the series as a whole and are probably only going to play one make it Black/White
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Fennec... :3
http://fennecfox.biz/wp-content/uploads/2010/08/Fennec-Fox-Having-Fun.jpg