Had this issue with a girl a long, long time ago...
ESB got her hooked. I'd go with that one. They love the romance.
I actually got to introduce my wife to the saga. Had her watch ANH and ESB first then showed her TPM and AOTC. She thought Anakin was a tool in AOTC but then ROTS hit theaters a month later and that movie totally won her over.
She thought the OT was "cute" but not really something to get too invested in.