Best IP addressing scheme for inter-networked organizations

What IP addressing scheme would be best to implement in a situation where different organizations are inter-networked?
Depending on the type of connecting made (Internet, VPN, PTP, etc.) the answer to this question could possibly vary. The IP addressing scheme that organizations use today is based on Ipv4. Companies use public IP addresses assigned to them by their ISP's or providers to communicate on the Internet. Typically, these public IP addresses are used on the outside of the organization. Through conversion (Firewalls, NAT, Proxy, etc.) organizations use lots of private IP addresses internally.

These internal IP addresses or private addresses were implemented by the IETF (Internet Engineering Task Force). This technique is documented in RFC 1918. Basically it defines three blocks of addresses:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

These addresses can be used by any and all organizations. Their only limitation is that they are non-Internet routable. However, these addresses can be used in one flat network or divided into multiple subnets. These subnets can be used to connect various organizational units, span geography, security, or ease administration.