What is the reason? Are you trying to buy something and turn around for a quick profit? Do you want to make an investment or a collection? How much money are you willing to spend? These are all questions you need to answer as well.
I do not collect a wide range of coins but there are certain ones over the last 20 years I wish I did. The 24k Gold Buffalo did some major jumps in price the first few weeks alone and within a few months seemed to double in price (especially in Ms69-70 condition) where as the gold presidential spouse coins have not jumped that much. Silver Eagles are always a good bet with the rise of silver lately, but you would have been better off buying them at least a year ago.
The problem is that there is no way to predict the future and which coin will be worth more than another, there are countless examples of this. How can a coin (A) come out with only 10,000 made be worth 2X where another coin (B) had ten times as many made and worth 6X?