if you had to eat at one restaurant for the rest of your life?

Schwartz's Hebrew Delicatessen in Montreal, home of the best smoked meat on Earth.

Arby's is a good choice, of course

for fast food / inexpensive

but if money were no object

then I'd definitely pick something higher-scale.

Probably a local Coney Island joint.

Golden Corral (or some better buffet) for the variety of food available. 

Arby's isn't necessarily a bad choice.

They DO have the meats... so the menu is diverse, and they have a variety of sides too.

I can't eat the same food everyday :-(

There is a Diner by my home that I quite enjoy. Or rather did before this nonsense. Takeout isn't the same.

Haven't been to one in over a year. The covid thing happened, and I'm a decent cook anyway, so I just got used to preparing all my own meals.

It would be the Restaurant at the End of the Universe.