36. Antichamber
Yet another one of the reality-bending first-person sci-fi puzzle exploration games, I know, but I think Antichamber is legitimately the one that sets the standard for them (aside from Portal, which is fantastic but isn't an indie game). If you enjoyed The Witness, I think you'll get a lot out of Anitchamber. It follows a similar structure of putting you in a big, mysterious place and letting you explore it freely at your own pace, and letting you slowly unlock/uncover new areas with constantly changing game mechanics. It's super challenging and even more trippy. It's difficult to explain exactly how the game works; there's like blocks, and barriers, and Witness-like little wall puzzles, and various guns to alter your block placement. Just a really cool experience.