From [1], it was said that
1) All users and providers of PPTP VPN solutions should immediately start migrating to a different VPN protocol. PPTP traffic should be considered unencrypted.
So, besides OpenVPN (since user need to install additional program), what would be the recommended choice if we use Ubuntu?
[1] https://www.cloudcracker.com/blog/2012/07/29/cracking-ms-chap-v2/