On the surface, it's a pretty easy answer for most people when asked what bike they would choose if they had to pick one to ride for the rest of their life. It's probably the bike they have now that they ride the most, but think about it...it's for the REST OF YOUR LIFE!
That's a long time. Every single ride, every turn of the pedals you do, it's your bike forever. And there are no take-backs, borrowing from a friend, or using something different. This is it. You can replace drivetrain components as they wear and service suspension, but the bike and its overall spec must stay the same from here on out.
There are plenty of things to take into consideration when choosing your forever bike. Versatility is key, of course, and then there's durability. Is it practical? Will you move and have different trails to ride? Will you reach a point in life where you want to do things at a different pace? You may think you're going to be riding the bike park at 60 or 70 years old, but let's be real, the only people doing that are the true legends, and even they would probably not be choosing a DH bike if it was their only option.
So, what bike would you choose? It has to be one from the list here and it isn't a weird or custom build or a situation where you can swap out wheels and tires. It's a current year model, off the floor of the bike shop. You're just stuck with what you pick and you're forever going to run what you brung.