GETVPN (Group Encrypted Transport VPN) is a Cisco proprietary technology that provides dynamic encryption for spoke-to-spoke communication over a private MPLS network. It does not require the maintenance of permanent tunnels, and it uses dynamically changing keys.
DMVPN (Dynamic Multipoint VPN) is another Cisco proprietary technology that can be used to create a VPN over a private MPLS network. However, DMVPN does require the maintenance of permanent tunnels, and it does not support dynamically changing keys.
GRE VPN (Generic Routing Encapsulation VPN) is a generic VPN technology that can be used over any type of network, including a private MPLS network. However, GRE VPN does not support dynamically changing keys.
Standard IPsec VPN is a generic VPN technology that can be used over any type of network, including a private MPLS network. It supports dynamically changing keys, but it requires the maintenance of permanent tunnels.
Therefore, the best choice for this environment is GETVPN. It meets all of the requirements, including dynamically changing keys, spoke-to-spoke communication, and no need for permanent tunnels.