Powerful isn’t exactly an ambiguous word.
Like what class could, if they so chose, cause the most problems lore-wise? Warlocks blew up a planet, but so did the void. Fel in general (not directly warlocks, but same magic school) was going to Halo the universe and was somewhat in the process of doing so and nuked several planets. But on Azeroth the strongest people are usually mages and sometimes a druid.
So, in the grander scheme of things, which class has the highest upper limit?