Least favorite part (spoilers)

[Abion47]

I'd disagree about Phi. At that point, the game is over, and the only thing left to do is launch the ARK. Sure, they could've made this a frantic sequence where you are racing against the increasingly growing WAU army as they charge down your doors, but I feel like that would undermine a lot of the tone that the game has been striving toward. Instead, Phi is meant as a place where Simon can finally have some peace of mind as he accomplishes his mission, gets on board the ARK, and launches into space, leaving the hell hole that is Pathos-II behind. It also serves as a nice point of stark contrast for the moment when Simon realizes that he isn't actually getting onto the ARK, that he is, in fact, trapped on Earth. It's one of a lot of factors that go into making that ending have the punch that it has.

To answer your question, though, my least favorite part of the game is the post-elevator tunnels of Theta. It's a dark maze where you have to follow the same formula over and over again - hide in the dark, find the way forward, hide from the proxies, solve a puzzle that involves making a lot of noise. It felt like the laziest part of the entire game, made even more tedious by it being the third time in a row you've had to deal with this kind of monster. It's in this part where you can really see the internal dichotomy that rages throughout SOMA's philosophical design come to a head - the split between whether to make an atmospheric horror (a la Amnesia) or a psychological thriller. These two directions are constantly at odds with each other throughout the game, so much so that when the focus switches from one to the other, the change is abrupt and drastic and it undermines the weight of the moment while you're recovering from the mental whiplash.