I find my brain extremely happy when a game provides ample opportunity to make connections, like in Dwarf Fortress, where I watch an event unfold, which can stir my creativity and imagination like nothing else. Writing a story out of it is extremely smooth and easy compared to other sandbox games.
I also find myself in love with immersive sims like Desu Ex and Thief, where level design and exploration take a front seat, every map is like a big playground with verticality and branching paths, where you find secrets and lore hidden around every corner in an atmospheric world.
What is immersion to you?
You can objectively measure it by asking a person playing for a fixed amount of time how much time has passed and measuring the discrepancy. Games that lately immersed me the most are Intravenous (1/2) and Riftbreaker. Also, Streets of Rogue coop with kids.