There is no single, most powerful spell. Meteor won't deal any damage to fire immunes. There might be a strongest combination of spells.
For me, the strongest sorc is the one who can solo all of the game. With respect to that definition, a fire/cold sorc (meteor plus fireball plus frozen orb) or a lightning/cold one (lightning and/or chain lightning plus frozen orb - I never tried one, I admit) with a merc who can keep Duriel and the Ancients off her and deal with the few bosses which are immune to the two chosen elements is the strongest sorc.
Sorcs specializing in both lightning and fire (including three element sorcs) are too weak in hell mode because they don't have enough skill points to make both lightning and fire spells strong enough to be effective.