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.
Yeah, also that.
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Xbox GT/PSN name: TatteredUniform
http://www.scuffletown.org/wp-content/uploads/2010/05/tRBE1.gif