MY opinion: The best silver to stack is that for which you will pay the lowest premium over the pure silver price.
If you are a silver stacker your primary interest is the price of silver, not the added numismatic value otherwise you have some degree of numismatic motivation. Nothing wrong with that.
From a purely silver stacker point of view the order of preference should be: 1. pure silver bars in the largest sizes possible, so that the premium per ounce over the silver is as low as possible 2. silver bullion coins - but not proofs, in lowest grade possible. 3. 90% silver coins, taken at face value, from CRH'ing activites (Why 3rd place? - because refining costs have to be paid.) 4. other bullion coins against which some premium has been paid, such as proof bullion coins. 5. World scrap silver coins. (That is the area in which I accumulate, because they usually come as "by" purchases that have been included in larger numismatic job lots)
I agree that bullion bars and rounds are the best. 100 oz bars aren't within my budget, so I would probably stick to 1 oz pieces and add to the stack frequently. The premium will be a little more, but I think 1 oz will be easier to offload than 100 oz bars.
For the price of 100 oz silver bars, you should probably consider gold, as well.
Tantum religio potuit suadere malorum. -Lucretius "Just because you're paranoid don't mean they're not after you." -Kurdt Kobain "Language is the source of misunderstandings." -Antoine de Saint-Exupéry My Want List: http://goccf.com/t/282022
Philosophically, the scrap silver that I have accumulated over the years, has all come as a zero cost in numismatic job lots, relative to the targeted numismatic item that I was interested in that job lot.
The actual cost of the scrap silver that I have accumulated can only be guessed at.
My opinion is that stacking constitutional sliver is the way to go. If the market for silver ever crashes (not saying it will) then you can at least spend the coins you've stacked and make back some of that money. Also, constitutional silver is still circulating around in some places, mostly in rolls, so it would be easier to obtain than bullion, and cheaper as well. Just my "two cents". :)
"Time is the coin of life. Only you can determine how it will be spent" -Carl Sandburg Some of my best/favorite finds:
Best silver is the largest bullion bar you can afford. The best bullion bar is the one without pretty pictures that you don't get attached to. The best bullion bars are square or rectangular so that they take up all of the room in your safe or hide-y hole. The bar should say what the product is, what the purity is, and how much it weighs. If your silver does not tell you what purity it is or what the weight is, then the next guy buying it will scrutinize it and maybe pass on it. That's not liquid. Sugar water isn't water.
I say this because I see bullion in the shape of skulls and whatnot, and I just feel that that style of stacking is for kids who are gripped by images and flashy stuff. Be smart. Pay the least amount possible. Purity should be .9999, otherwise you have to smelt the coin to get your actual silver. US coins are dumb because they rely on a face value for worth, and they never weigh the same from coin to coin, regardless of how many silverbugs can recognize silver (most people can't and are not aware that silver coins once circulated). Honestly, you'll be spending those coins at face value because most people just don't care enough to know what silver is. Nostalgia is one thing, business is another.
I have also heard that 1000 oz bars are not as easy to sell because they often have to be refined first to ensure purity. Since these are normally industrial units of silver, if you end up with one and try to sell it then it would be considered outside the chain of integrity and sent off for refining prior to sale.