TITLE: What are some very important papers published in non-top journals?
QUESTION [121 upvotes]: There has already been a question about important papers that were initially rejected. Many of the answers were very interesting. The question is here.
My concern in this question is slightly different. In the course of a discussion I am having, the question has come up of the extent to which the perceived quality of a journal is a good reflection of the quality of its papers. The suggestion has been made that because authors tend to submit their best work to the best journals, that makes it easy for those journals to select papers that are on average of a high standard, but it doesn't necessarily solve the reverse problem -- that they miss other papers that are also very important. (Note that the situation more generally in science is different, because there is a tendency for prestigious journals to value papers that make exciting claims, and not to check too hard that those claims are actually correct. So there one has errors of Type I and Type II, so to speak.) 
I am therefore interested to know of examples of papers that are very important, but are published in middle-ranking journals. I am more interested in recent papers than in historical examples, since it is the current journal system that we are discussing.
Just in case it doesn't go without saying, please do not nominate a paper that you yourself have written...

REPLY [2 votes]: There are a couple of examples already of papers only "published" on the arXiv; here is another one.
"Total positivity, Grassmannians, and networks" by Alexander Postnikov https://arxiv.org/abs/math/0609764, preprint from 2006.
In this paper Postnikov introduces the positroid cell decomposition of the totally nonnegative Grassmannian. The totally nonnegative Grassmannian and its positroid stratification have become an important topic in many diverse areas, from cluter algebras to scattering amplitudes in physics. In particular, the main combinatorial tool introduced by Postnikov here, namely, plabic graphs, has been applied to everything from soliton solutions of integrable systems (https://arxiv.org/abs/1105.4170) to knot theory and sympletic geometry (https://arxiv.org/abs/1512.08942). (Incidentally, for a textbook treatment of the theory of plabic graphs, see Chapter 7 - https://arxiv.org/abs/2106.02160 - of the book-in-progress on cluster algebras by Fomin, Williams, and Zelevinsky.)
According to Google scholar this paper has been cited over 500 times, which is pretty good for 15 years. I am not totally sure why this paper was never published (even though, full disclaimer, Postnikov was my PhD adviser). But it is my understanding that it was passed around as a manuscript many years before being put on the arXiv, and he was asked to put it on the arXiv and make it publicly available before his tenure review. Considering its impact, I certainly think it could have been published in a top journal.