Hardest to sell because nobody wanted it was Allied Leisure Dynomite. Those early Allied Leisure games are hard to find in full working order. I brought it to at least 3 different shows for free play, but still it was hard to find a buyer. I eventually sold it for way less than what I had into it. Oh well. I don't think I'll get any more Gen 1 Allied Leisure games in the future, even though I basically wrote the book on game repair for them (on pinwiki). At least that should help people who are interested in those games.
So far, I haven't really had to part with any games I was particularly attached to. I've done big project clean-outs a couple times, so there are a few games I never had a chance to fix up and play. Two that I would've liked to finish were Star Race and Bounty Hunter. For Star Race, I just ran out of space and something had to go. For Bounty Hunter, it was basically a wreck and pile of parts that I picked up somewhere and just didn't want to deal with in that condition. Everything else I either just lost interest in, or finished, had my fun, and then sold.
Right now, I do have a stargate prototype whitewood up for sale that I've held onto for several years. It was tough to come to the decision to list that one for sale since it's one of a kind and such an interesting design. But considering I haven't done much with it, I figure it's time to let someone else have a crack at it and free up space for something else.