ALSO in case it wasn't clear, I suppose I should make sure you're familiar with the idea that Pokemon games are often sold as "twins" as I often call them. It basically means that you'll have two games that are almost exactly the same with only a few differences (usually just involving which Pokemon you can capture, and even then, only a handful of species are different between the two). So for example when I say to get "Firered/Leafgreen" I don't mean you should get both, but rather pick one.

For sake of simplicity, I'll just recommend that you go with whichever one you like the box art for the most.

OH WHICH REMINDS ME I'd also recommend Emerald if you want to play through a storyline similar to that of the recently released Alpha Sapphire and Omega Ruby. 

So my top recommendations are Emerald, Firered/Leafgreen and Heartgold/Soulsilver, in no particular order...