For instance: 'diagnosis' gives back > 7 million results and that is roughly 1/3 of all the citations. I know that I am probably asking an 'irrelevant' search question as I am after the biggest scope, the inverse of relevant search.
all [filter] will give you all
Using the publication date between 1000 and 2011: http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=1000%3A2011[PDAT]
Results: 1 to 20 of 20541957
no , see how to use the NCBI EUtils http://eutils.ncbi.nlm.nih.gov/
If you query for 'publication formats' you get essentially all of PubMed back. The reason is that their automatic query turns extends this query to "publication formats"[Publication Type] OR "publication formats"[All Fields]. Almost entry in PubMed has a publication type, which is a child term of the MeSH term 'Publication Formats'.
The reason why 'diagnosis' gives as many results as it does is much the same. The query expansion turns it into "diagnosis"[Subheading] OR "diagnosis"[All Fields] OR "diagnosis"[MeSH Terms]. The latter MeSH term is one of the top-level terms.
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I can see what you're getting at here, but I think a better way to state it is "what are the commonest terms in PubMed", rather than "which query". A query can be anything you like including, as shown by the answers, a query that returns everything. What the query does is match some terms in the database (title, abstract, MeSH etc.) Lars' answer explains this well.
sorry, just for curiosity: why are you asking this question?
you're right Neil, that's what I was after. Not really looking for tricks but for the commonest domain-specific terms. It answers Giovanni's curiosity too.