It is disregarded for income tax purposes.  Disregard it.  Use the owner's ID number.

It is not disregarded for payroll or excise tax.  Use the LLC's EIN.

In regards to W-9s (and therefore receiving 1099s), the W-9 instructions say:

"If you are a single--member LLC that is disregarded as an entity separate from its owner, enter the owner’s SSN (or EIN, if the owner has one)."

And this IRS webpage says:

"For federal income tax purposes, a single-member LLC classified as a disregarded entity generally must use the owner's social security number (SSN) or employer identification number (EIN) for all information returns and reporting related to income tax. For example, if a disregarded entity LLC that is owned by an individual is required to provide a Form W-9, Request for Taxpayer Identification Number (TIN) and Certification, the W-9 should provide the owner’s SSN or EIN, not the LLC’s EIN."

https://www.irs.gov/businesses/small-businesses-self-employed/single-member-limited-liability-compan...