Greatness, Semantics, and Response to SHO
Mark S.

Your argument is fair enough -- if one cares to define the meaning of "greatness" with an exclusion of moral discretionary power. But I think that is not a valid exercise in this case for two reasons.

Firstly, words change their meanings with time.

Who, today, thinks that all 'faggots' are pieces of wood, all 'gay' people are just happy, all 'intercourse' is merely conversation, and 'blue' is just a colour? Clearly, words change their meanings with time, they acquire depths, shades of meaning and even homonymic characteristics. Thus, all of today's dictionaries already becoming historical relics, whilst language is a vibrant living continuity.

Secondly, in regard to sentient beings, the adjective "great" must have the element of moral eminence. Few people today would press any argument that Hitler was Germany's "greatest" politician, despite his achievement of restoring Germany's international prestige by bringing Germany out of its financial and emotional depressions of the 1920s and 1930s.

If "greatness" applies only to 'power', 'potentials','potency' or 'material capacities' (as Tolkien seems to have used it) then you're quite correct; Feanor is the greatest.

And, correspondingly, Melkor is the greatest agent in Middle Earth short of the enigmatic (and largely silent) Iluvatar who can (and did) change the world, geophysically on two occasions.

But, let it be remembered, Tolkien wrote LOTR and The Silmarillion with a purposely archaicised English. His "great" was not necessarily our "great".

So perhaps I have a specious definition of "great" and "greatness", but not one that I think is indefensible.  If "great" can be used with your dictionaries' (current) definitions, then Madonna, Cher, Jennifer Anniston, Tom Cruise and Mick Jagger are all "great" sentient beings largely because they are excellent, rich, influential and celebrated entertainers!