From my POV, nothing is really "broken" and it's unrealistic to ever consider that. Are guns broken? Well, indeed, compared to swords and arrows, they are. Yet they came into existence and use. Are nuclear weapons broken? Probably. Didn't prevent them from existing, though. In fact, any new creation can be broken compared to former ones: it's called progress.
So, are some spells and professions more powerful than others? Probably. Though, it also depends on the context: in the land of the blind, the one-eyed man is king after all. In the end, people adapt to everything: an invisible flying mage throwing fireballs may be powerful... the first few years. But after some time, not only would there be so many of them running (well, flying) around, having one in one's army wouldn't give such an edge, countermeasures would have been developed and anti-air devices (i.e. magic items) able to detect invisible would be widespread as well. After all, if profession X is so much better than the others, then every rich family will push one of their heirs to become one... And if SuperWarriorClass is way better than any other fighter class, then after some time, anyone able to become a SuperWarriorClass will be one, so being a SuperWarriorClass wouldn't give a party such an edge, as most other party would have a couple of SuperWarriorClass members as well!
So, keep in mind I'll talk about the case where the profession or spell list is still relatively new, scarce and not widespread.
Anyway, so, the professions I consider the most powerful are:
* in second the Sage (RoCo. II) because, well, he can pretty much read anything written in a flash (just turning the pages), deduce (correlate) about any fact, has a permanent +20 to ME and RE (yes, the closed mentalism spell list "Mind mastery" gives a +50 bonus but they are active spells with a duration that need to cast whereas the Sage's spells are subconscious spells that don't use any power point and have a constant duration; in other words, cast it once, subconsciously and without spending PPs to boot, and, there, constant effect giving +20 to ME and RS!) and are incredibly proficient to anything that involves speaking (therefore manipulating people and whole crowds; I mean, +75 to public speaking where the Sage has already a +3 profession bonus and a low cost of 1/4!) So, what, you upset the puny Sage because you're a strong fighter. Sure, wait for him to take ten minutes to persuade the whole city you're THE one to be killed, including everyone in the nobility and clergy! Not to mention to deduce your every move from just a few pieces of information. And since he's using, you know, skills, there are no RRs: it's just a matter of him rolling high... which isn't very hard. Come on, the powerful fighter can kill a guy in one blow? The powerful magician can kill a dozen in one spell? This guy can start a civil war or a holy war in one speech! And yet...
* the absolutely most powerful: the Enchanter from RoCo IV. Because he has the manipulative talent of the Sage, but does so from the shadows, using illusions, deceptions, etc. The Sage knows how to speak; the Enchanter knows how to speak and socially act, including and foremost through seduction. He's the power behind the throne, using skills and spells to hide his true self, create illusions to deceive others, manipulate and control through mental control, charm spells and actual seduction, being able to learn people's secrets to better manipulate them, with the combat abilities the Sage lacks (through the Command Words list... though not many would even think to be aggressive in his presence, anyway, as he has knowledge of the Calm Spirits list). So, sure, the powerful fighter or mage may be able to fight armies, this guy may raise an empire... and hire the aforementioned fighter and mage, or merely manipulate them to do his biddings.
* special mention to the Theocratist from the Alc.C. because it's a minor, not quite widespread companion. Aside the fact his Bless spells can grant bonuses up to +70 at level 20 (+70!!!), he's a religious equivalent to the Sage/Enchanter as being a crowd and personal manipulative type (up to +30 to propaganda, public speaking and seduction after all) and the incredibly powerful ability to convert anyone to his religious views with just a one-minute "sermon" (at level 20)!
There, the classes I think any GM should keep very rare, since those guys can start whole (civil / holy) wars quite easily. With tactics an academic skill and leadership a social skill, the Theocratist and especially the Enchanter may even become great military leaders whereas the Sage may be a great tactician, being able to read the every moves of the enemy, leading them to be great conquerers and empire-builders.