If you could choose only one game...

Either Platinum, HGSS, or B2W2. All of those games have tons of postgame content, and a lot of replayability.

This is quite a difficult question. I think I'd have to go with Emerald because of the Battle frontier.

It would be pokemon XD: Gale of darkness

Probably Gold. It's my favorite game, and I would have a hard time giving it up permanently in favor of something else. (I would also consider HGSS for the improvements, but there's something about the nostalgia of Gold that I love.)

I would certainly miss the features that the newer games have added, but there's something I find really relaxing about playing the first two generations of games that isn't there in anything newer. I prefer the stat exp system (only have to worry about base stats, then train Pokemon however you want without having to go through a tedious EV training process first), and there's something charming about the graphics. My favorite Pokemon are also from the first two generations, so I would probably be ok on that front. I suspect I would miss modern breeding and such, but if I could only play one game, nostalgia would probably win out.

Either Mystery Dungeon: Explorers of Time & Darkness (either one) for DS or Stadium for N64. Depends on if I have other people nearby that will play with me.

This is a hard choice for me. I have experienced every main pokemon game release as intended during it's active generation. Pokemon R/B/Y while being nostalgic and great I couldn't play for the rest of my life due to the horrible graphics compared to now a days plus the lack of pokemon, moves and game mechanics I've grown custom to having. Same reason can be said for S/G/C, LF/FR, R/S/E

With those being took out of the equation then I'd have to say the oldest games I wouldn't mind playing for the rest of my life would be D/P/Platinum however if I had to pick a 4th gen game it would be HG/SS for it still has 4th generation pokemon and features but gives me the nostalgic of Pokemon Yellow Pikachu were pokemon are able to follow you and then being a remake of G/S/C and bring me back into my childhood days, as well as plenty of post game content.

pokemon sun/moon.
it has good story for me, plus i can hunt shiny in "a little bit" soft way by SOS. i spent almost the whole year to hunt shiny in this game and it was very fun. i even nearly close to complete the regional Alola Pokedex. i'm sure i can spend more time hunting shiny with this game.

Crystal, a million times Crystal. I've never had as much fun playing or replaying a Pokemon game as I've had with Crystal.

Definitely HeartGold/SoulSilver.

It has loads of content and delightful features I just really love, two whole regions, and just a really nice general atmosphere. It's honestly pretty much as close to my ideal Pok?mon game as the series has gotten thus far.