As much as I love Amnesia, I think Frictional's best output has been Penumbra. The sense of "going deeper" and unraveling the mystery behind the story was phenomenal. I also think the addition of Combat made each encounter more tense and unpredictable compared to Amnesia.
It's too early to say anything about SOMA so far, although I'm worried about the fact that the protagonist speaks and comments on his situation... It may break the immersion... Or maybe I'm used to Penumbra/Amnesia's format.
It does seem to have a great atmosphere, as expected.