I can't decide which is my most-hated category of rejection. Yes, getting no response or a form letter is no fun, but it happens to everyone, I expect it, and I don't get too bent out of shape.
What causes me to scratch my head is when I get something in the "it's not you, it's me" category. (Ever notice how querying is sort of like dating ...) "Your book sounds very interesting and I'm sure it would be of great use to many people, but I don't think I could successfully place it with a publisher." ("You will make some lucky agent happy some day ...") If it is interesting and useful, why can't it be placed?
However, what causes me to mutter to myself are folks who ask for partials or fulls and then reject for reasons that should have been apparent to them after they read the query, such as how long the book is, what it contains, etc. Nothing like mailing a hard copy of the entire book to someone (who said it was too long to attach to an email) and then not having it read at all because she thinks the book's word count is way too high. Unless it is hearing that someone was "disappointed" to see certain content in the book, when that content is specified in the query. Do I have to put warnings in bold in my query, specifying the word count, reiterating the content, etc.
OK, I've vented, so I feel better.
(Oh, I forgot another category: those who have folks they can recommend to improve your writing, not that they can specify what is wrong with your writing. One agent offered to recommend a "professional writer" to help me with my proposal. She hadn't even
received my proposal

Given the timing, and what someone else said about her rejecting but not opening the submission involved, I'm not even sure she looked at my partial.)