The most broken jutsu? Replacement. It was so broken that Kishi stopped using it.
Kakashi's Kamui was fine until he started spamming it and apparently forgot the other 998 jutsu he knew.
Totsuka and Yata are last resort jutsu, or should be. It's just that people often use it to prove that Itachi solos.
FTG does have its limits, and I think Izanami is a cool concept.
I mostly dislike needlessly elaborate jutsu that seem like they could be simplified/optimized without losing their actual purpose/use, or can be easily replaced with more effective jutsu.
For example, why does Sage Art: Thousand Hands (name?) have such elaborate shape? Why couldn't it be just a wall?
Or, why use Mind Transfer when there is Mind Disturbance/Destruction/whatever which doesn't leave the user helpless?