Define "long term" storage.
Best is a cutter top tin, but they've not bee made since the late 1970's
Square and Rectangular tins are goodfor maybe 7-8 years before they start to become unreliable, with the percentages of failures increasing with the years. Keep in mind that almost none of them are hermetically sealed from the get go, and you can smell the scent of tobacco leaking from them. In fairness, they weren't designed as a long term storage solution.
Round tins hold up better, as the seal pressure is evenly distributed around the perimeter.
I've seen far more round tins that were still holding up after 20 years than square tins.
Food grade Mylar should be a good option, especially in the 7 mil variety. Push out all the air you can and heat seal.
Vacuum sealing of Mylar or glass jars will slow down the aging process if that is what you want.
Then, you just have to hope that other bad things, like mold, won't get to your stash.