In your opinion What is the best WAN Technology ? and why?

MPLS is the way forward, it encompasses all the advantages of Frame Relay, ATM and IP Routing. Whether it's QoS, Scalability,  Traffic Engineering or Security.

MPLS (Multi-Protocol Label Switchng)

It depends on your topology.

People use various technologies like HDLC,PPP,Frame Relay,ATM,MPLS etc.

Day by day MPLS is becoming more popular.Following link will give you some idea about MPLS:

http://searchenterprisewan.techtarget.com/feature/MPLS-vs-Ethernet-Which-WAN-connectivity-option-is-best

You cannot geenralise which technology to use. It depends upon various factors like business need, bandwidth availability, relaibility of service provider. It also depends upon the business usability of the business.. Like triple play ie, Data, voice and video. In the present era, MPLS networks give the value of money without performance degradation!!