Star Wars Imperial Shuttle #7624 for me. Kind of what got me started in investing heavier into Lego sets. Amazon had a Buy-2-get-1-free deal years ago. I think I would up with around 35 of them. Started selling them in the $160 range but kept rising to $215 area at the time. Have two left that will be listed eventually.
10179's gotta be the one for single set profit if you go by per set amount. Had 11 that I bought for between $250(1) and $407(10), started selling too early, so a couple sold at $1200, then 1400, then 1800. Still have 3 left though.
You just can't get caught up in the "if I still had it" anxiety. Just think about it that you were able to either enjoy or reinvest those funds into something else that continued your investment.
Probably a few others that have the same ballpark rate, but haven't started selling yet. GG's, Flagships, Emerald Nights, etc.