The best autographs to buy depend on whether you are purchasing them to make you money or whether you require them for a personal collection. There is a different criteria to follow depending on where you want this to lead.
If you require autographs for your personal collection then price is not really important. The main criteria to investigate are whether the autographs are real and not fake. You also need to try to get autographs that are not dedicated to someone. Dedications decrease the value of an autograph in a collection or for sale. The best autographs to buy will be the ones that are genuine and not dedicated. Price is not essentially important as you may be happy to pay large sums of money for an autograph for a personal collection that will give you great pleasure and ultimately grow in value in the future. I have autographs I have paid lots of money for because I really wanted them for my collection. The main thing is that you like it and want it.
The best autographs to buy for investment are when you either have a chance to buy something rare that it will grow in value over the years or buy something of an up an coming celebrity on the chance of them becoming very collectible in the future. It is more risky gambling on this but if you get in early on the career of someone who becomes very successful the profit can be extremely high. It also follows that as well as a celebrity becoming successful autograph prices also rocket in the event of the sad death of a much loved celebrity.
Finally the best autographs to buy for profit making are ones that are at a very low price, sometimes from a collection, if you buy the entire collection and split it down or getting a good deal by researching what sells well. If you are buying for profit you have to make sure that the item is at a price that you can resell for profit or that it is such a rare autograph that you know it will definit