Elminster has said himself that he is not hte most powerful mage in Fareun, but I would say he is more powerful than all of the other chosen of Mystra (including the Symbul. This because he is Mystra's favourite, he is the first and oldest chosen and he actually beat the Symbul once when they first met (correct me on that point if I'm wrong)However, I think Larloch is the most powerful. He is over 2000 years old, he is a lich, he has a few of the Nether scrolls (2 of which Mystra herself crafted and gave to him!!!) and he has in any way most experience of all mages. When the chosen of mystra first appeared he studied them for a long time until he fully understood their purpose and then he decided to leave them alone (killing a chosen would most likely have some sort of negative effects on the weave, which would make Larloch's research much more difficult). On top of this, as mentioned before, Larloch has no political interests whatsoever, he only concentrates on developing his magic (another reason to why he is the most powerful). If anyone wants to know more, go to
Larloch - Forgotten Realms Wiki. There you can read about his immunities, tactics and so on.
One more point considering this discussion however. During periods like the time of troubles, all mages were quite powerless. Just something to be considered I think...