I'd say Sonic Youth's choice in gear has varied FAR too wildly the past twenty years to honestly identify a particular "best" example.
Jazzmasters are the obvious choice, as they have consistently been the most popular choice for both Thurston and Lee, but I'd hardly call SY the "quintessential Jazzmaster tone"
IMO, the key to SY's sound has much more to do with alternate tunings, volume, and pickups than anything else.
That said, buy a Jazzmaster. There are affordable choices out there---more than ever before.