While I could say Red/Green/Blue/Yellow, I don't want to give in to nostalgia. Rather, I'd say Diamond and Pearl because I can utilize Pal Park to obtain my favorite 1st gen Pokemon, and not have to deal with the 4th gen Pokemon I find very lame and unimaginative while also having acess to Wi-Fi features like the Global Trade Station.
Add to that a much more complex battle system, stat calculation, and physical/special attacks being sorted in a way that makes sense, and I'd say it's pretty great.
Pokemon Platinum solves my only problem with the game- no random battling over Wi-Fi; I assume it was left out of Diamond&Pearl to push sales for Battle Revolution.