What makes a particular gold coin the "best" to buy depends on what you want. A skilled numismatist might be looking for piece with particular historical value. Someone else might just want something that looks nice. But generally speaking, the best coin to buy for investment purposes, is the one that offers the best value.
Identify weight and purity. The most important contributing factor to a coin's value is its gold content. This can be determined by identifying its weight and purity. Most bullion will have both of these features clearly marked, but a legal tender coin often will not. With a little research, however, it should not be difficult to find the weight and purity. Multiplying these two figures yields the gold content (for example, a 1oz coin of .999 purity contains .999, essentially 1 oz. of gold.
Add in additional costs. When evaluating the cost of a coin, don't forget to add hidden costs such as ,tax, shipping or other costs associated with the purchase. For each coin you're comparing add these additional costs to the asking price to get the total cost of buying the coin.
Calculate per ounce cost. Divide the total cost by the gold content. Be sure your gold content number is in ounces (convert before dividing if necessary). For a 2oz coin, divide by 2. For a 1/10 oz coin, divide by 0.10. The coin with the lowest per ounce total cost is generally the best value.
Consider intangible qualities. Other factors besides the gold content that can affect a gold coin's value include the reputation of the mint that produced the coin (national mint products are especially highly regarded), the age or rarity of the coin and its physical condition. Considering the relative value of these factors can sometimes make a coin with a higher per ounce cost the better buy in the long run.
Easily recognizable coins and those in good condition often sell at a premium over other coins. Other coins however, with roughly the same melt value can be the better buy compared with coins without any particular historical or collectible value.
Coins with considerable historical value or that are rare will have value far in excess of their intrinsic gold content. Be sure to confirm a coin's rarity or historical value and the demand for the coin before assuming it's a good buy.