I've never regretted accepting an article I've accepted. I've never gone back and said I should have rejected that article.
I've never regretted a revise-and-resubmit. in almost all cases, the author has taken my suggestions and published the article. In almost all cases the author has seemed grateful or at least graceful.
The only question, then, is whether I have regretted an outright rejection, a case where the article might have been more easily fixable than I gave it credit for. There is no way to really make this judgment, because it's a subjective measure of where the threshold lies. I've come to realize the system of peer review is rather flawed, and that I've thought of it as fine simply because I assume everyone does it like I do, with prompt, fair, helpful, and thorough reviews. Unfortunately, that's not the case.