I think you are asking the wrong question, as it is a little ambigious.
rewording your question "Which ingot do you feel has the most sales" or "Which ingot sells for the most markup"..
As for fact, I have no idea - the only ingot I've seen is iron from killing some argonauts.
In my opinion though, the ingot with the highest volume of sales is the most common one (thus has the least markup), but the most popular ingot (as in desirability) is the rarest ingot which consequently will have the highest markup.
So quickly jumping to entropiawiki.com, I see the the most common ingots (least markup) might be Lysterium, Copper and Frigulite, and the most desirable ingots could be Tridenite, Rugaritz, and Xeremite.