I agree with Goomboo that if your looking for an old version, yellow beats blue and red (which also work on the Gameboy Advanced). Gameboy advanced games also work on the DS but I don't think a 3DS, and out of those games I like Emerald/Saphire/Ruby as well because there is a twist in the way the story is, although Leafgreen/Firered are good too. For DS games (which I think work on the 3DS) I would go with Heartgold/Soulsilver, although I like aspects of Diamond/Pearl too, but personally Platinum is one of the games at the bottom of my list of Pokemon games.
If it were me and I didn't have any of the games I would go with Firered/Leafgreen (depending on the system).