My dad is thinking about taking a 1 oz gold coin in payment for an item he sells that is around $1800. He was told he could pick from a gold eagle, gold maple leaf, or south african krugerrand. I don't know anything about gold coins so I don't know which he should choose. Do they have value beyond the gold content? Should he worry about the condition of the coins, year, etc...? I don't have a lot of info on the particular coins beyond their names.
Gold Eagle hands down. Any value beyond melt would be judged by year and grade of the coin or how well the coin looks.
In the US I would say American Eagle, anywhere else in the world I would say the Maple Leaf. US collectors love our coins, but the higher purity of the Maple Leaf makes it more popular worldwide. Krugerrands have always sold at the lowest premium versus gold as long as I have been around coins. I don't know if it is the purity, the former government or what, but it was always the cheapest.
The gold eagle would be first choice, and I wouldn't take a damaged or greatly worn coin. But I wouldn't reject a Maple Leaf or Kruggerand if it meant losing business.
I prefer Maple Leaves for the 99.99% purity as has been mentioned already. There are even some that have 99.999% as well. I've heard that people like Krugerands because of the quality of gold from that area of the world, if there is such a thing. Personally I don't like the color of a Krug. They look kind of dirty compared to a sparkling US or Canadian gold coin. The other thing I like about Maple Leaves is that they often come in sealed plastic so you don't have to worry as much about the condition of the coin or how you're going to store it.
You can't go wrong with any of these coins but if you are in the USA, take the AGE first, Krugerrand 2nd. They are very easy to sell. Without knowing more specifics, there isn't anyway to tell you if there is a numismatic value beyond the metal price.
Does your father know his buyer pretty well? There are certainly honest and legitimate reasons why someone would want to pay for an item with an appropriate amount of gold, but counterfeiting of all three types of coins gets reported quite regularly.