virtualbox.org

What's the last version that works with Windows 7, Windows XP, Windows 98 and Windows 95? And what host Operating System do I need to have for those?

Since you mention the 'host' in a separate question, I'll guess that you're inquiring about what Virtualbox version still supports 7, XP, 98, and 95 as guests, or VMs.

7 and XP are still officially supported as VMs in the latest as of today 6.1.32. There is a new glitch regarding Guest Additions going on now for 7 Vista and XP, due to recent SHA256 upgrading of security hashing protocols by the world's standards makers. Dropping back a few minor versions for Guest Additions will probably get you a GAs version that will install, see https://download.virtualbox.org/virtualbox/ , or for 7 web-search how to upgrade to SHA256 security hashing.

98 and 95 have never technically been officially supported, though they do run, and we have received no indication that 98 and 95 do not work in 6.1.32 anymore. (98 and 95 have never had Guest Additions, but there are workarounds. Peruse the forum's How-Tos and Tutorials section.)

For the host OS, it does not matter. Choose whatever OS you are familiar with. Note that 7 is no longer supported as a host OS.