Ninja Gaiden 1 and Ninja Gaiden Black for the original Xbox, along with Ninja Gaiden 2 for the Xbox 360 are the ONLY versions you need to play.
Stay away from the Sigma versions, unless you prefer a watered-down experience. The Sigma versions are simply too easy, in my opinion. Although, they're not as bad as Ninja Gaiden 3. What a nightmare!
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