If you are looking to invest in silver, best bang for the buck are either bars like Johnson Matthey or generic rounds. They have the lowest premium over spot. But if you are looking for something that is easy to liquidate, probably US Silver Eagles but the premium on them is pretty high but you do get some back on sale.

Also 90% pre 65 silver coins are good too but not the graded stuff as obviously there is a collectors premium on those.