If you'd get one only I'd recomment the third main one Ultimate Ninja Storm 3, you must have watched the anime before so playing the third one first won't spoil anything on you.
But if you want the full story I'd recommend getting all of them. And the fourth one too... STORM series is that great for anime fighting game. Better than anything Dragon Ball in my opinion.
Thanks! Hmmm... have you played the fourth one at all? If it's better I might just buy that.
Haven't played it.. but yeah, according to what I have read It must be better than the third one.