Regarding all the round sheet metal duct work exiting our typical gas
fired,
hot air heating
furnace:

What's the best way to seal these duct joints, at least where one can
actually see them ?

Duct tape doesn't really sound like the best way to go.

Duct tape is exactly what you want... It's metal and shiny silver and has a
wax paper strip on the sticky side. You do NOT want "DUCK" tape, which is
grey cloth tape and really sticky/gooey.