They all seem to be the same size (3G). What's the difference?
texlive-20160523would refer to the official, first release of TeX Live 2016 that was slated for June 6th, 2016. The date of May 23rd, 2016 probably references when inclusions into TeX Live 2016 was halted/frozen.
texlive2016may refer to a current bundling of the entire TeX Live 2016 distribution. This may differ from the above as packages may have been updated since the official release.
texlive.isoprovides an packages version of
texlive-20160523specific for burning to a disc.
You can download either one of the above and install it. Once fully installed, you can push an update to make sure you have the latest and greatest.