If you're my age, maybe it's your knees

It sounds like you've done a very comprehensive attempt at eliminating sources of a creak. I'd check the frame over inch by inch to make sure there are no cracks. Cracked crank? Any recent changes to components?

Does the creak happen all the time or just when you are pedaling?
Does it matter how much force you applying to the pedals? Is it worse on steep climbs or during sprints?
Does it creak when you hit a bump or rough patch of road?
Can you recreate the creak by bouncing your weight on the bike while standing still?
" " " " " " by pulling forward and back on the handlebars while standing still with the brakes locked?