What is a stretchy cast-on for top down socks?
The long-tail cast-on is a sturdy one, excellent for when you need structure, but not so much when you need stretchy. Better choices, in turns of stretch, include:


cabled cast-on (super easy)
tubular cast-on (to go with 1-1 ribbing)
rolled edge (easy, durable, and would let you still use the long-tail method)


The links are for TECHknitting, but these are well known techniques and a web search will bring up lots of resources for any of them.