First, DO NOT start with the 1st or second generations made for the Game Boy and Game Boy Color. The remakes of these games on the GBA and DS are superior options, especially for newcomers. Also, DO NOT play something like Black/White 2, which is the oddball direct sequel.
I feel that generally, out of the DS and 3DS games, your best bet would be X/Y or AlphaSapphire/OmegaRuby. Along with being the most recent and thus polished games, they are successful at what they try to do.They both also contain the best mix of Pokemon to capture and battle.
I feel that X/Y is a bit better for somebody who wants to explore an in-game world for the first time, and contains better side content. AS/OR has more late-game content, and a better story in terms of enhancing the game's flow, but takes place in a less interesting setting for first-time players.
I would not reccomend Diamond. Pearl, or Platinum because these games move at a fairly slow pace without much benefit compared to other entries. I personally love Black/White, but those are best played after other games because of their specific themes in terms of design and storytelling. HeartGold/SoulSilver is full of content, but lacks the sophisication of more recent games.
