While Tomonobu Itagaki adores Grand Theft Auto IV, he totally despises Ninja Gaiden Sigma. And Itagaki's own Team Ninja made it! From an interview with website CVG:
What did you think of the PS3 game Ninja Gaiden: Sigma? I'm told you won't even sign copies of the game...?
Itagaki: It was no good.
Why's that? I thought it was pretty good.
Itagaki: All you need to do is play Ninja Gaiden II and then try and play Sigma afterwards and you'll see why. Despite the fact that it's on a 'next-generation' console there's no evolution whatsoever. That was made not by me, but one of my sub-ordinates who basically tried to copy the success of Ninja Gaiden.
Ouch. We thought it was pretty good! Elsewhere in the interview, Itagaki confirms that Ninja Gaiden II will never appear on the PS3, and if it ever did that would be "a loss of face for everyone involved."