There isn't any such thing as the "best" VPN software.
First off, there are two pieces: the VPN client and the VPN server. As mentioned above, there is an RFC for VPN, however most server vendors provide extra features if you use their client as well. Interoperability varies as well, and may depend on the exact configuration of the switch, in addition to which client you're using.
(note: VPN server is also known as: VPN Switch, VPN Concentrator)
For example: We use vendor Nxxxxx, and disable all non-Nxxxxx VPN clients from being able to connect (on the switch) That's a security feature we wanted to configure. It can support some 500 concurrent sessions, and also supports PKI-based authentication.
I don't know if those are important features for you. It works (mostly) for us.