What's the latest version of Safari?
The latest version of Safari is: 11.0
Safari on OS X
Safari comes installed on all Apple computers, so you shouldn't need to download it. To make sure that it's up to date, use the Software Update feature by clicking the Apple logo in the very top left of the screen, choosing "Software Update" and following the instructions.
Apple used to provide a few different versions which were all the "latest version" for that platform; eg Maverick's version was the Safari 7 branch, Yosemite's version was the Safari 8 branch but they were all the "latest versions"... As of September 2015, Apple now seem to be keeping the one branch (Safari 9) across the few most recent versions of OS X.
Safari on Windows
Apple don't make or support Safari for Windows any more. The last version of Safari for Windows was 5.17 (released in May 2012). Apple no longer provide it for downloading. There are much more up to date browsers for Windows, including Internet Explorer, Chrome, Firefox and Opera.
Safari on iOS
Apple make Safari for iPhones, iPads and iPods. As with OS X; it is a default app and cannot be removed. To make sure that it's up to date, you should keep your device's copy of iOS up to date, because doing this will ensure that Safari is up to date as well.