The forgery WAS so exquisite that it hung in the museum for the past two decades without discovery.
The correct tense to complete the sentence is past simple so the best verb is was.
The main part of the sentence that provides as the hint about which tense to use is the phrase "the past two decades." It refers to the past.
The simple past tense, sometimes refered to as the preterite, is used to talk about a completed action in a time before now. The simple past is the basic form of past tense.