You would use the W-2 that shows the vesting of the shares. From TomD8: the per share cost basis of your RSU's is any lot created by the vesting divided by the GROSS number of shares you received in that lot.
You will need to consult any records you have for the issuance of the RSUs to find the number of shares. Please see this discussion in TurboTax for tips to determine the cost basis of the RSU.
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