My bet for the absolute tight, long time hermetization of bottles/jugs:
My favorite cheese is the "Babybel" from Holland.
Small 2" or 2 1/2 dia cheeses, having purple colored 1/8" thick vax coating.
This type of vax is not rigid: can be melted easily by hotgun, in a spoon. Before turning the cap on, immerse the thread of bottle, turn on the cap, and heat again.
(If you cannot find "Babybel" in your grocery/supermarket, other vaxes can be useful... )
Istvan