The hardest to sell (theoretically, as it isn't sold) would be LOTR. It was the first pin I ever personally owned and it was a surprise Christmas present from my wife in '03.
The hardest that I did sell, wasn't actually hard at the time but was an absolutely mint inside and out TAFG. I would look at it and tend to just see stacks of cash sitting there instead of a pinball machine and that temptation eventually got the better of me. I have had 5 TAF/TAFG's go through my hands as I worked my way up to that one and that was the absolute best one. I very much regret it because that title is what got me into pinball in the first place back in the 90's at our local nickel arcade. Now I am desperately hoping for a TAF remake from CGC but obviously know that is a long shot.
The hardest to sell due to lack of interest in the market was Ironman. I had to drop it way down in price from an already reasonable price and it took like 6+ months to sell.