Gold/Silver/Crystal will always hold a special place. Just loved how finishing the Johto region unlocked Kanto. I've always kind of hoped we'd have a game where you start in a new place, but after completing that league, you can travel to all the leagues from prior games and challenge their gym leaders and elite four.
In terms of actual game w/ features and everything, I'd have to say B/W2.
Jury is still out on gen 6 til I beat it.
1 is great for nostalgia, but it's a pretty tough game to go back and enjoy, atleast for me. Too much stuff has been introduced since those days.