The newer ones improve on the old game mechanics, and they keep shifting the power balance on which pokemon and types are great. So if you start a new one, the older one will most likely feel dated.
Personally, I'd go with either RBY and continue onwards, or get BW and BW 2 (later this month)...
Actually, now that I think about it, FR/LG is a good place to start too, since you can transfer your pokes to the next gen. RBY and GSC can't transfer their poke to gen 3 onwards. If you care about that, that is.
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My Little Phineas and Ferb: Summer is Magic!
Aww, I lost to SuperNiceDog, Winner of the Rivalry Rumble Guru Contest