I don't know that other one, but what hooked people on MSFS was the AI terrain made from satellite images letting you fly from anywhere to anywhere, and some capitals are handmade.
They also added missions based on the previous games to celebrate the series history.
Oh, it's a game now? Back when I dabbled with it (early 2000s) it was a simulator as the name says. Flightgear is an open source simulator that, according to pilots, feels more realistic.
I mean, yes, as I’m sure you know already. Flightgear is a fine product and lots of respect to the contributors, but the support around MSFS, the level of detail and whole host of other factors make MSFS the one to beat; even if the flight model of XPlane is probably a tad better.