RollerCoaster Tycoon 2, open-sourced and still expanding.
OpenRCT2 is an open-source re-implementation of RollerCoaster Tycoon 2 with modern multiplayer, UI improvements, and an active park-building community. It needs the original RCT2 data files, then unlocks decades of quality-of-life that the classic never had.
Minimum
RCT2 data files · 2 GHz CPU · 2 GB RAM · OpenGL 2.0 · 500 MB + RCT2 assets
Recommended
RCT2 data files · 3 GHz CPU · 4 GB RAM · Any modern GPU · 2 GB storage