TdeV - Windows, OS/X and Linux - see
http://calibre-ebook.com/download - the Portable is for Windows
But I don't see how that greatly effects what tablet to buy, calibre itself does not run on either of the major Tablet/Phone operating systems - Android, IOS. There are 'app's and calibre plug-ins that facilitate integration between calibre on the desktop and tablets, they are desktop OS agnostic (I think).
BR