I’ve been posed the same question about ‘if you could only listen to one band’ and I usually decide that if I were restricted to just one, I’d sooner drop the hobby altogether. Anything loses its luster after too much repetition for me.
But for the sake of actually answering the question, would have to be something that can’t just be shut down, has variety in and of itself, and stands the test of time. So maybe something along the lines of Fallout 4? I know Skyrim is the darling but Fallout 4 has more actual stuff to fiddle with between base building, constant procedural questing, etc. I dunno. That’s not a perfect answer, but it’s in the same realm at least.
I was raised more on SimCity 3000 but yeah, absolutely no head for finances. Take out every loan, obtain all the money, do whatever you want, and then be shocked when you’re bankrupt because suddenly you had to pay all those loans off.