I wouldnt say izanagi because its too risky. Most people have only two eyes (apart from danzou lol) and after you cast izanagi twice, thats it you are blind. I believe that in order to measure the strongest ninjutsu, you should also take into consideration how risky it is.
Id say it has to be uchia shisu technique 'koto amatsu kati'. The ability to control somebody without them even knowing. You can simply avoid combat and just cast this technique on your opponent to kill themselves and that's it you won.
Others include susanoo, amaterasu and tail beast bomb.
Btw i do not believe that edo tensei is the strongest. Its the most annoying technique but not the strongest