Windows 10 Tech Preview can be easily be installed in VMware vSphere 5.5 (ESXi 5.5). Even the latest version of Windows OS is not listed in the supported OS.
The installation of Windows 10 including VMware tools is without a problem and everything works great.
Microsoft has created Windows Insider Program to all users to test the new version Windows 10. You will have to agree to Microsoft Terms and conditions to download it.
Windows 10 is supported on 5.5 U3. It is just that the Windows 10 option is not available from the drop down. Which is why we advise you to select windows 8.
From vsphere 6.0 onward you can see the windows 10 option in the dropdown.
Nevertheless, you can still deploy and run win10 on 5.5 U3
Why do vmware not add "windows 10" and "windows 2012 R2" as an option?
It is a easy way, to get a view of how many machines you got of each windows type.
It can't be that difficult to add...
With 5.5 it is now available only via the Embedded Host Client Fling.
Otherwise it is only 6.x