What are the most common Layer 2 WAN technologies? Also, what Cisco certification will be of most help regarding this technology?
You won't found ATM in any of the current Cisco exams (except for the CCIE SP and a little bit in the CCIP MPLS exam), but you'll find frame-relay in CCNA, CCNP, and even CCIE R&S, formerly there was a CCNP exam called Remote Access, but it has retired and i believe that it was replaced with this one:
Implementing Secure Converged Wide Area Networks
which covers MPLS and other stuff (some ATM and some FR), please take a look at it.