Which kernel version is best for real time?

The thing is a newer kernel will only have better performance if there has been significant changes in the area that are being used in your application. Changes to filesystems you don't use, drivers you don't or in architectures you don't use don't expect any improvements.
To be really sure you have to read change logs and stuff like that to be sure. Newer may very well be the same as what was supplied with the Linuxcnc ISO.
Oh and don't run any other network services that you use for your motion controller, in your case you should have a dedicated NIC for Remora and another for everything else.
I still don't know why people insist on using 3rd party remote desktop clients when X is actually was designed from networking from the beginning and there are plenty of available clients and servers in the distro repos that have been time tested.
Running a CNC machine remotely is a another kettle of fish. If you need to do remote admin use SSH and learn to get comfortable with the command line.