Sure, if you are reading the word "arguably" completely literally, but that's not the colloquially understood implication of the term when used to describe a work. Obviously the author of the parent comment is implying they they would possibly consider it the best PKD novel, but the colloquial meaning implied when someone uses the word arguably, generally isn't just to describe one's own opinion, but a significant portion of the popular or critical consensus.

One person could take a position opposed to the general held consensus on any topic, but if one person is the only one to hold this opinion, in english, it would generally not be described as a position that is "arguably" the case, even though if you read the word literally, one person is technically arguing it.

Also, I asked because I wanted to get the above user's opinion on the matter, not your dismissive comment which isn't contributing anything. I've read the VALIS trilogy, but I've never heard any of VALIS trilogy novels described as possibly PKD's best work.