As of writing, all Vitas/PSTVs are fully hackable. For firmwares 3.65 up to 3.72, H-Encore2 can be used to run native homebrew and plugins, and optionally downgrade to either 3.60 or 3.65 if you wish to install HENkaku Enso and run homebrew and plugins straight from bootup.
3.60 is considered the absolute golden firmware to be on, as almost all plugins and homebrews have been developed to work on this particular version. And as well, you will be able to install a permanent hack which will persist on every reboot, called Ensō. 3.65 also has the same level of maintainence as 3.60 in terms of homebrew and plugins (and support for Ensō), though there might be a couple of old plugins that were not updated for it. However there are some updated plugins, Vitacheat Z06 being one of them, which will only function correctly on 3.65, therefore it's ultimately your choice as to which of the two firmwares you wish to proceed to and use.
(And if you're wondering how you will be able to play games on 3.60/65 which require higher firmware versions, there is a plugin called reF00D which enables just that, provided you fill in the necessary encryption keys)