The project is focused in Poland, but for a project of this scale it is common (and reasonable) to distribute the workload. So I’m sure there will be parts of it being done in the US.

According to Steam the top 3 are:

1. Oxygen not included
2. Civilization 5
3. Factorio

But that of course does not include the games not running from Steam and pre-Steam games. So World of Warcraft is somewhere in there too. And the final Top 1 must be Transport Tycoon Deluxe (even if you don’t include OpenTTD).