What types of gold bullion coins (or bars) are generally sold with lowest premium over spot and which dealers generally sell for the lowest premium over spot, for small quantity sales?
My local bullion shop has the lowest premiums on Krugerrands, on the other hand some online shops such as APMEX sells them as some of their highest premium coins.
If you're looking at modern, bullion denominated in troy ounces, your best bet is to compare prices on:
Krugerrands
Canadian Gold Maple Leafs
Austrian Philharmonics
American Gold Eagles
Mexican Libertads
Chinese Pandas
That should be about the order from cheapest to most expensive. But then it depends again on the coin shop.
If you're looking at older coins which aren't in troy ounces, but still are gold. Some of the lower premium ones are:
British Sovereigns (and half sovereigns)
Austrian Coronas
Austrian Ducats
French/Swiss 20 Franc pieces
The premiums on these can vary dramatically depending on the coin shop, but often you can find average circulated pieces selling for $5-25 over the spot price, less than an American eagle or a Krugerrand even! The downside is they aren't very easy to know the gold content (for example a half sovereign is .1177 of a troy ounce) and they aren't always the most pure coins (they are usually made from 22K gold to make them resistant to wear). You also get the bonus of owning a coin that has collector value beyond bullion due to the fact that for most of them they don't make them anymore (the Austrian coins are an exception, they continue to mint coins with a fixed date such as the 1780 silver Maria Theresa Thaler and the 1915 gold ducat).
I forgot to mention Mexican gold coins, which are generally quite affordable and come in a range of sizes. They are also worth checking out.