[CentOS] Optimal VPN

I am looking for the optimal VPN. Well it doens't have to be that elaborate. Just the best VPN. We currently have some customers using PPTP, some using openvpn, some using Cisco Any Connect and there are a few others.

So my question is, if you have control of both ends (client and server) what is the best VPN to use? There are not too many requirements, but a big one is

The VPN must return the same IP address to the same user each time

That is there must be a specific IP address assigned to a user/password combination. pptp does not really do this but I wrote sort of a backend (or maybe frontend? ;-) ) to change the IP address assigned based on a login and password. It is extra stuff I would prefer not to do though.