I'm sorry to go so off topic. I will try to clear this up. Currently we've been discussing who is the worst failure as a person in terms of individual goals, but we haven't addressed your original question as failures as shinobi - rather than just people.
What is the goal of a shinobi? This issue has been brought up a few times, and I think is epitomized by Naruto's "ninja way." The goal of any shinobi then would be to follow the shinobi way or code. The biggest failure as a ninja is the one that betrays the ninja code the worst then.
The current official code of the shinobi are to be military tools for their respective villages, and to carry out missions with success at sometimes high costs.
Thus, you could say the biggest failures as shinobi are Akatsuki, who have betrayed and abandoned their village that they were duty bound to work for, for personal gain. Orochimaru and Sauske can be included too.
But then again, that's just my interpretation of what it means to be a shinobi, and as you can see in the Naruto universe, this interpretation can vary widely from ninja to ninja - so getting a clear cut answer won't be easy.