Aporia: Beyond the Valley
A beautiful adventure game focused on lonely exploration of gorgeous environments and solving some intriguing puzzles of medium to easy difficulty. If those two elements sound good to you, then you will enjoy this game. Note, however, that this is not entirely a calm "walking simulation," and indeed there is a section in which you are hunted.
The game's strength is its environments, which are lushly designed and make good use of stunning, dramatic lighting. There is a simple story, told subtly with animated vignettes, which weaves like a thread through the game world. Much is made in the marketing material about this being a "game without dialog," but that's really overblown. Firstly, there are many games without dialog, so it's hardly innovative. And secondly, the vignettes are fairly explicit. Though there's no spoken dialog or written text, there is definitely narration in Aporia.
The game is not perfect. For one, it suffers from erratic pacing. Suddenly there are complex puzzles. Suddenly you need to stop and look for hidden items. It's as if each section is a different kind of gameplay, which feels too regimented and incoherent. Worst is the "you are being hunted" section, which is jarring and poorly implemented. I wish there were an option to disable it and let you play with just exploration at your own pace.
Another annoyance is that this game can be frustrating for completionists. Explorers like to explore everything, but Aporia has far too many collectibles. To get all the achievements you would have to comb the environments with unsatisfying thoroughness. There's usually nothing clever or delightful about these items: they're just there somewhere on the map. Missed any? You'll have to replay the game from the beginning and try again, because there's no going back to previous sections.
Finally, as others have noted, Aporia does not run smoothly even on powerful GPUs. My GTX 980 struggled to maintain a constant framerate at 1080p.
For me, these problems were not serious enough to detract from the beauty of the experience. I strongly recommend Aporia.