Technically speaking, a dedicated leased line ( point to point copper/fiber) is the most secure. Your switching technologies like ATM or MPLS( ATM on steroids) use switches along the data pathway where someone *could* get at the packets. When I worked spook projects, the only secure link was a hard line from A to B and a scrambler box.
How secure your data is will depend more on the encryption you use then the transport method.
Dont forget the weakest link in the security is the person at the other end.
MikeS
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin