Syl started a discussion last week on what she might choose if she was only going to play a single video game for the rest of her life. Telwyn followed suit.
It's actually something I've pondered, idly, a few times over the years, along with that perennial favorite "if you had to live inside an MMO, which one would it be?". For me, the answer to both questions is probably the same: EQ2.
It's not that EQ2 is the best MMO, although it's certainly one of my personal top three favorites. It's more that it's so many different games all bundled up together. It almost feels like cheating to pick it as the only game I'd play.
There's such a portmanteau of ideas, all layered thickly, often chaotically, across each other that it runs far greater risk of overwhelming than boring any player, new or old. It would take a very long time indeed to understand, let alone exhaust, them all.
For example, Crafting in EQ2 is a fully realized MMO all of its own, complete with storylines, solo and group content, quests, gear progression, even "dungeons". So much development time has gone into adding so much content it could quite literally be released as a stand-alone game by now.
In what you'd usually call the main part of the game, Adventuring, there's a plethora of classes - it sometimes feels like dozens of them - each of which plays significantly differently from the next. Famously - or infamously - every one of those classes has enough spells or spell-like abilities to fill forty or fifty hotbar slots. If that's not variety enough, there are over twenty races, each with its own flavor - although why you'd want to play anything other than a ratonga...
You can start in half a dozen different zones and level up in many, many more. There's absolutely no need to follow the same, worn path each time, although over the years you will certainly develop preferences. You can race to max level and back-fill your AAs or set the slider to any point on the scale you like (always assuming you've ponied up your sub for the privilege), allowing you considerable control and variation in the leveling process.
Like crafting, leveling in EQ2 is a full game - provided you like making multiple characters - but at the top of the level curve the endgame waits with its minutiae of never-ending incremental improvements for those that care for that kind of thing. That's a Zeno's Paradox you'll never outrun.
For pure explorers things aren't quite so infinitely extensible. You will inevitably run out of new places to see before the developers can patch more in. Still, while Norrath after the moon exploded isn't as indescribably huge as the original sprawling Norrath of EverQuest Sr. (itself an extremely good candidate for the One Game) it's big enough for you to have forgotten the start by the time you come round to the end.
Of all the games-within-a-game that would keep me going indefinitely, though, I think the clincher is Housing. Housing in EQ2 ranges from the simple decoration of a basic inn room to the construction via break-out of entire new zones. Indeed, decorating and building are tantamount to two separate games in themselves, especially once permitted player-made third-party tools like the Layout Designer come into play.
If EQ2 was the only game I had to play maybe I'd learn to use that editor. Mrs Bhagpuss did and she created wonders. I just filled all my houses with stuff. Of course I'd carry on doing that, too.
So, yes, if I had to settle on just one game to play for the rest of my game-playing life I think it would have to be EQ2. If Vanguard was still going and growing I might have gone for that instead, since it had all of EQ2's attractions plus Diplomacy as well, but sadly that option's no longer on the gaming table.
EQ would be my second choice. It, too, shares most of EQ2's benefits and adds plenty more of its own but EQ2 is just a more comfortable place to hang out for extended periods, somehow. I guess the only other contender would be GW2, in which it seems I can spend thousands of hours despite there really not being all that much in the way of variety.
Pushed to a choice, though, I'll take EQ2 as my One Game