Because it is leveraged to silver, I suspect that SLW is probably the best bet if you think silver will continue to rise. (I think it is going much, much higher). You don't have to become a coin expert to buy this.
The cheapest silver are the bags of pre-1965 silver coins. Halves, dimes and quarters. If you are investing money into physical silver, buy these FIRST! They are 'real' money and they will protect you from inflation and deflation. You don't have to become a coin expert to buy this.
After that, I would buy silver Eagles and Maple Leafs. These will be popular with collectors in the future. Do not buy the super high grades at premium prices! You don't have to become a coin expert to buy this.
If you want to collect coins and you have a good budget collect the early American silver coins. 1790,s - 1810 silver dollars. These are truely rare.
Morgan's and Peace dollars are not rare except in super grades or certain mint marks.
Personally, I like liberty $20 gold pieces in MS60 or above.
Collecting rare silver coins is collecting art, not silver, so why limit your "most undervalued coin query" to silver coins only? The gold, silver, and bronze/copper coins are all in competition with each other. If you want an eye-opening experience, find a 1990 or so coin price redbook and compare those prices to todays. Then ask yourself if any of those prices appear undervalued compared to what silver bullion has done over the same timeframe.
If you believe silver prices will rise significantly from here, rare silver coins may not be the place to be. Might even be a bad place.