1. "every US silver dollar collector should own at least one ______"
Morgan - afterall it IS affordable.
2. "every world silver coin collector should own at least one______"
Perhaps any trade dollar / yen. This was the best indication of global trade and often involves several countries.
3. "every world coin collector should own at least one _______"
While I am tempted to say a Krause, as a catalogue is VERY useful, if it had to be a particular coin, I would have to say, set of coins minted in various metals. (why not, it's very affordable, excluding precious metals and odd metals)
4. "every world gold coin collector should own at least one _______"
One chervonetz.

Afterall, it was a denomination supposedly minted for trade purposes. Except it failed hard

Somewhat.

5. "every high-end investor/collector should strive to find a _______"
A market to them to stick to. STOP RAISING THE PRICES ELSEWHERE!!! Or perhaps they should stick to kilo gold coins.