Depends how you define strongest? By difficulty of battle or by power of the HNM?

Prov Watcher would easily top the charts though the availability of proc lock and fanatics spam significantly lowers the difficulty then what it would otherwise be.

After that, Mul Legion NMs.

Then Rex / Ig-Alima for the same reason as PW.

Then possibly ADL depending. He is by far the cheapest NM of them all though not necessary the strongest or hardest. We rely on stuns and PD (best groups can do it without) to bypass his cheapness.