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?
By this metric, Oblivion was right in the top 3 slot for me. Started playing Friday straight after work, and ended up being late for work Monday morning, no sleep.
In that period also nearly burned my house down - got hungry, put noodles on pot, went back to play for 5 minutes while they cooked. Got hungry, went to put noodles on, saw that I already did that and they were on fire.