I don't necessarily think they are the best, they are just the only ones I have experience with as I am fairly new to investing in silver. APMEX is the only person I've used besides a few smaller pieces on eBay. But, they are the only ones I can personally account for when purchasing.
APMEX typically has the largest inventory and to penny's point they are extremely reliable. Personally I think one might pay a higher margin over spot when using APMEX but at these levels silver is a gift that not very many will be able to get in on
A number of my purchases have come out of APMEX but I also like Provident Metals and Gainesville Coins as well. GLTU
News 
Market Data 
Discover 
