As you may know, Kernel 4.1 has been released last week and the development for Kernel 4.2 has started, the first release candidate version being scheduled to arrive in a few days.
Linus Torvalds has announced on his Google+ account that Kernel 4.2 will be bigger (in number of commits) than Kernel 4.1 and 4.0, the kernel developers and contributors being now in the middle of the merge window, this meaning that many improvements are ready but they have not been submitted yet.
For now, Kernel 4.1 LTS is the most advanced stable kernel available, and can be easily installed on Ubuntu and derivatives. Just follow the instructions from this article.
As a reminder, starting with Kernel 4.0, the long awaited Life Patching feature has been implemented, permitting the users to update the kernel without having to reboot the system. While this is not important for regular users, this feature is very good for Linux servers.