I'm assuming you are referring to free versions only.
I still use VLC and have been happy with it but now that I am usung Media Monkey to organize my music, I find it much more convenient to use its built in player instead. VLC is far more popular than UMPlayer (never even heard of it) so I would suggest using VLC.
Until recently, AVG free was bad news. I was happy with Avast free until it recently became too top heavy with extra bells and whistles. I've been using MSE
(Micro$oft Security Essentials) since last June and have been happy with it.
I have a scanner that rquires a full version of Adobe Acrobat Standard ('tis expensive, btw) and I have been spoiled by what it is able to do. However, if all you need is a free reader, Foxit
is safer than Adobe Acrobat Reader and has a good reputation. If you need a free virtual printer to make PDFs, PDFCreator
is a good choice. I still use it for making simple PDFs since it's faster than Adobe Acrobat.
I never heard of XnView but I frequently use IrfanView and love it. Its an excellent program fro mass filename renaming, and simple mass picture editing. I also use it for downloading photos from a camera card since it works much better than the POS wizard that came with Windows 7. It can be used as a picture viewer but I prefer to use the picture viewer that came with Office 2010 (2007's is also good) over the POS viewer that came with Win 7.
I personally prefer IE9 over Firefox but Firefox is far more popular than IE. Google Chrome is also popular but I personally don't trust Google.
I use 7-Zip
for unzipping downloaded files that need it. I haven't tried the other two since 7-Zip meets my needs.
Hope this helps you some.