Mass Effect 1.
The first encounter with the Reapers, with Sovereign, is such an incredible moment. These horrifying, unkillable deities that can't be reasoned with or stopped that just want to destroy was so intimidating on the first playthrough.
The reveal about the mass relays and how they guided tech advancement was incredible too.
You exist because we allow it. You will end because we demand it.
Same here. The story and presentation are so sublime. ME universe is was designed so comprehensively and coherently it left me awestruck on many times throughout the first playthrough.
And the sequel is just a cherry on top, drawing from the now-estabilished world, themes, interstellar politics, all to build one absolutely epic finale. No other game managed to engage me so much.