For CHM files: Kchmviewer has tons more features than Sumatra PDF and is WAY better than Calibre. In fact, if you open .CHM files in Calibre with anything other than the separate ebook-viewer.exe program, it will default to the standard Windows viewer (the same result as if you opened it on a new Windows install).
For EPUB files: SumatraPDF followed by Calibre. Both are way ahead of STDU Viwer and KchmViewer.
For MOBI files: again, SumatraPDF followed by Calibre. STDU Viewer results were mixed.
For PDFs: SumatraPDF all the way, though STDU has a few nice features. Despite it's converter capabilities, forget about opening PDF files in Calibre, even via ebook-viewer.exe.
Conclusion: I expected STDU Viewer to dominate but obviously the big winner here was SumatraPDF: the soft color background and by-default two-column format worked well. I also expected a lot of these programs to render similarly via publicly available tools (three of them are open source) but that's not the case at all.
Other notes:
I was also very pleased to see that no closed formats such as Micrsoft's DOC, DOCX, XPS, etc. are being used by various ebook sources.
Strangely, despite not viewing CHM files well, Calibre's CHM -> EPUB converter works well. Ultimately Calibre should probably contain a copy of Kchmviewer, which could actually happen as they both have a GPLv3 license.
I'd like to comment that I use to create PDF files by PUBLISHING TO PDF from a well known vector drawing app and I could tell by the naked eye that SUMATRAPDF shows the best quality of it on screen and outputs the best quality on the printer among all the pdf readers I tried.
I've found that when viewing epubs, particularly the non-fiction kind with pictures, caption formatting and whatnot, that SumatraPDF is vastly inferior to Calibre. I downloaded some epubs on photography a while back and unwittingly opened them in Sumatra, only to find that either these were some of the most amateurly designed epubs in the world or Sumatra was to blame. Needless to say, from that day on Calibre became my default epub viewer.
This is not to dispute your opinion, but merely my experience.
