I'm not sure what you mean by "consider synonyms," but to me, the most negative stanza in this poem is the last one (although I should say that the whole poem feels quite negative to me).
I say this is the most negative because it seems to express such loneliness and isolation and fear. He says that what is inside him is much scarier than anything outside -- more frightening than the vacuum of outer space, more frightening than dark fields that make him feel alone. This is a very bleak idea.
The previous post was quite accurate in suggesting that the entire poem has a strongly negative feel to it. I think the third stanza contains the most imagery that seems negative to me. Much of this might be how it effects me, and like all questions of this nature, you will have to assess for yourself which stanza compels you to feel which emotion. Yet, I think this particular stanza contains many images that effect me. The bleakness implied with the last two lines is fairly powerful in terms of connoting negative imagery. A setting where there is little expression and even less to express indicates a type of wasteland that is quite brutal in terms of placing myself in that setting. The isolation, abandonment, and notion of emptiness seems to indicate much in the way of negativity.