Operating systems?

Android only because it uses less resources. If tablet ever become super powerful, then any OS will be fine.

you don't really have a choice. if you get an iPad it comes with iOS. otherwise, you get Windows with a Microsoft Surface or Android with everything else.

if you're asking which is BETTER, that is a matter of opinion. they ALL do the same stuff, so it is usually best to stick with what you already know (or what you can afford). none of them is really better than the other.

now, there ARE different levels of speed in Android tablets. i like the Samsung Galaxy Tab A's myself. nice balance of speed and price.