Way I see it, Hayabusa's evolution can be divided into phases.
1) Birth - This was the time of the original NES games. He's pretty cool, but also kinda clumsy and easy to trick. Generic protagonist.
2) Rebirth - This was the time when Itagaki first revived the character for DOA, spanning DOA1 through DOA3, DOA2U and the first Ninja Gaiden. He became something of a cryptic taciturn, speaking in riddles and metaphors. In this time, he is an absolute badass. He does what needs to be done, because he can do it. Because he's the best damn ninja ever. This is my favorite period.
3) Evolution - Spanning the time between DOA4 and NG2, Hayabusa becomes less cryptic and more direct. Some of his cool, mysterious aura is lost. That said, it's during this period that he really starts to define his role as the Singular Super Ninja. He is absolutely unstoppable, and the canon universe begins to shit its collective pants at the mention of his name. It's still a cool time for Hayabusa, even if some of his awesomeness was lost.
4) Decline - Sadly, when Itagki left, things do seem to go downhill. He retains some of his aura from the Evolution stage in Dimensions, DOA5 and WO3, but he's also kinda silly and comedic. Then, when it's his time to be a badass and save the world, he just becomes a whiny, incompetent bitch (NG3) with generic protagonist syndrome. His words are uninspired, his actions leave much to be desired and his charisma is lower than either of the previous two phases.
He's still Hayabusa and he's still awesome, don't get me wrong. But he's definitely seen a decline in recent times, imo.