Even before he very briefly played it for an Indie Games Marathon, my pick was Hollow Knight, for the following reasons;
-Phil sucks at Metroidvanias in general, getting hoplessly lost and repeatedly dying early in Super Metroid, a much easier, more straightforward title.
-HK doesn't hold your hand in terms of exploration and where to go next. Hell, you don't even start out with a map and have to find the cartographer and buy a map for him in each new area. Phil would get hopelessly lost and have to beg chat for where to go next every time.
-Deaths have real consequence, in terms of potentially losing geo, a portion of one's soul meter, as well as having to backtrack long distances from the last save point, particularly punishing when one is facing a boss. Phil would throw a temper tantrum and possibly ragequit over this alone.
-Fails look particularly awful and inept in this game.
-Very difficult to simply cheese bosses, and healing, unlike other games, isn't instantaneous but risky.
-Just the right level of difficulty, at least not taking into account the Godhome DLC. There are some hard challenges, but none are overwhelming and unfair, and it's wedged between far more manageable sections. In other words, hard enough for Phil to lose time and again, but not so hard that the audience watching wouldn't do better.
In typical Phil fashion, he only played the very beginning portions, before there is a real challenge, and given that he knew where to go, correctly named several collectibles and enemies whose names are not intuitive, and most suspiciously, instantly guessed the Soul Shade could be used to kill an Elder Baldur guarding the entrance to Greenpath, he almost certainly looked up some information beforehand.
None of that would help him later in the game, however.