#2 Eddie Guerrero (1997 and 2005)
Rey Mysterio was unmasked by good friend Eddie Guerrero not once but twice.
The first time was in 1997 on an episode of WCW Monday Nitro. Rey Mysterio was facing 'The Man of a 1000 holds' Dean Malenko in a singles match. Mysterio had just hit Maleno with a springboard hurracanrana and was in the midst of pinning him when Guerrero came in from ringside and pulled the mask right off Mysterio's head.
Malenko locked in the Texas Cloverleaf on Mysterio who was hiding his face. Mysterio immediately tapped with his only objective being hiding his face from the cameras.
Guerrero unmasked Mysterio for the second time on an episode of WWE SmackDown in 2005. It looked like Guerrero was coming out to rescue Mysterio from a beating at the hands of MNM but he turned on Mysterio after chasing away the then-SmackDown Tag Champs.
Guerrero culminated his vicious attack by tearing Mysterio's mask off.