From a story perspective, it does not matter which one you play first. The only exception is that you should play Black/White before Black 2/White 2. From a difficulty perspective, XY or ORAS first. Those are the easiest. From a gameplay mechanics perspective, don't touch anything before Generation 4. And then instead of Diamond/Pearl, go with Platinum. Save Generation 5 for last because you will more appreciate its deep story if you do. I would say start with ORAS, XY (unless you would rather wait for Z), HGSS, or Platinum. Then try Black/White followed immediately by Black 2/White 2, and make sure the sequel you get is the same color as the first one.
However, if you simply want to play the greatest Pokemon games ever, go with Colosseum, followed by XD: Gale of Darkness.
P.S. From the OP, I assume you have a 3DS, which means you can play everything from Generation 4 onward. Do you also have a GBA or a DS (OG or Lite)? Because if not, there is no point in anyone advocating you start with anything before Generation 4.
NNID: garretslarrity
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