I like this idea a lot. I think it could be fun to do voting at the end of every major release cycle, for excellence in dwarfening during the previous "age." Alternatively, annual or bi-annual voting. But that way there can be some new stuff in with all the great old forts. There could be several "laureates" for each category, and one winner.
I also wouldn't limit it to just Fortress mode (which I would separate from Legends). Not only should some Adventurers get love, but the programmers of the utilities we all take for granted deserve a little extra mention too. Here are some of the categories I would use:
Fortress mode
Mason's Award - excellence in above-ground fortressing (the Flarechannels Prize)
Miner's Award - excellence in below-ground fortressing (the Undergrotto Prize)
Mechanic's Award - excellence in complicated feats of engineering (or computing)
Siege Engineer's Award - excellence in trapmaking and otherwise murdering invaders
Stonecrafter's Award - excellence in stone sculpting, above or below ground
Architect's Award - excellence in aesthetic fortress beauty
Pump Operator's Award - most creative or effective use of magma
Alchemist's Award - greatest contribution to Dwarven science
Butcher's Award - to the fortress exhibiting the most callous disregard for life of any kind (this one makes me kind of nervous)
Woodcutter's Award - best handling of the Elven menace
Axe Lord's Award - greatest military, either individually or overall
King of the Dwarves - best or coolest overall dwarf of the era
-others that people think of-
and finally,
Mountainhome Award - best overall fortress of the era
Adventurer Awards - I don't know, I don't play this much.
Utilities
Could possibly give awards for: Increasing playability, Modding, Graphics, and general hacking, plus many others I'm sure.