Manifold Garden
A masterpiece. If you're a fan of first-person puzzle games, this is a must play.
Portal opened the door to the genre, Talos Principle enriched it, but Manifold Garden distills the formula to its beautiful, raw fundamentals. There is no story here per se, nor flashy graphics, just a pure atmospheric experience that drenches you in wide-open geometry and ambient music.
The puzzles are many and wonderful, having you play with colors, blocks, streams, gravity redirection, and mind-bending non-Euclidean spaces, which present themselves to you gradually and thoughtfully. There's no handholding. Instead exploration and experimentation will yield the many a-ha moments. I managed to complete the game without any guide, and though I often felt challenged I never felt the solutions were unfair or obscure. Think, try, and think again. And be rewarded.
I was fully engrossed and smiling from beginning to end. What a marvelous achievement.