I started with APMEX, went to JM Bullion, and ended up with Bullion Exchanges. All are reputable dealers, but Bullion Exchanges had the lowest premiums for the stuff I bought. They also put silver and gold American Eagles on sale frequently where the premiums are even lower.
I no longer buy from the first two because, to me, they weren't competitive when I was buying more silver. That could have changed, so you should still shop them.