While looking around for an answer to this, I have been getting conflicting opinions/answers, so I decided to ask here: Is publishing a great research paper as an undergrad more important than great letters from your professors when applying to grad school?
Another phrasing: If you had only one of these (great research vs. great letters), which scenario would more likely help the graduate application?
Short answer: In practice, letters are more important.
Long answer: Doing great research is great, but if I'm on an admissions committee, how do I know your research is great? I can read your paper, but unless it's close to my area of expertise, it can be hard or at least time-consuming to determine its quality. I probably have too many applicants who wrote a paper to read everyone's paper in detail (depending on how common undergraduate authorship is in my field). And even if I'm convinced the paper is great, how do I know what your contribution to the paper was? (In the fields I'm familiar with, it's very rare for undergrads to author solo papers.) On the other hand, if I have a letter from a faculty member whose word I trust saying that this paper is great and you did most of the hard work (or even that he/she is very impressed with your potential), that's much more useful.