Too many good ones to pick just one
Went to a garage sale near where I used to live back in 2010. It was advertised on Craigslist and there was some PS1 games in the pic, nothing crazy, but stuff I wanted. I make sure to get there right when it starts and am the first one there. Immediately start looking for PS1 games and to my surprise there's a huge stack of about 20 of them. First game is Tekken 3, next is FF8, the freakin Valkyrie Profile, then Ogre Battle, then Legend of Dragoon, then Ehgiez, and so many other heavy hitters and excellent titles. I ask how much and the older couple selling them says $2 a game. I buy the whole stack and walk away. This was the find that got me really into game hunting.
A local used media store that was owned by Go Hastings used to be a goldmine for me for several years. My best finds from there were getting a bunch of rare Dreamcast and Gamecube games that just got traded. I picked them up at $3 a piece and it had games like Mega Man X Collection, Phantasy Star Online Collection, Spawn (Dreamcast), Skies of Arcadia Legends, Chibi Robo, and a bunch of other great games. Another amazing score actually happened well after the employees caught on and started sniping all the expensive games that came in. There was a bunch of Saturn games that made it out to the shelf including Panzer Dragoon 1 and 2, Winning Post, Mega Man X4, Saturn Bomberman, and about 6 other good Saturn titles. Again, $3 a piece. That store went out of business in 2016 and I miss it so much despite it starting to suck towards the end.
Went to a large local flea market on a Friday which I'd never done before. It was mostly a ghost town with about 1/10th the typical amount of sellers. I hadn't found anything until I came across a guy selling an Ice Blue N6 console with about 25 games. Almost all the games were desireable titles including all your must haves, but also all Mario Party 3, Ogre Battle 64, Harvest Moon 64, and Conker's Bad Fur Day. I ask him how much he wants for the whole box and he says $60. I don't even try and talk him own on it. I found SO MUCH stuff at that flea market from like 2010 until 2013. Unfortunately it really sucks now, but it used to be incredible.
Early on in my game hunting days I used to go to an independent thrift store in a rough part of town. They had a glass display case similar to what Goodwill has and occasionally I'd find some retro games in there, typically priced no more than $5 a game. I check it out one afternoon during the week and immediately see a mix of loose and boxed SNES games. I ended up getting Earthbound loose, Breath of Fire 2 CIB, and about 4 other pretty good SNES games for a mix of $5 and $10 a game.
We have a local used media store called 2nd and Charles that has several locations in my state. When one of the stores first opened my wife and I went to check it out. Behind the glass case were several CIB SNES games, all in really good shape. Chrono Trigger, Secret of Mana, FF3, and a few others were among them. The price tags were on the other sides of the box so I had to ask to see them. I was expecting them to be insanely expensive, but too my surprise they'd priced them as if they were loose and also slightly lower than that. I ended up getting those three games and a few other good ones all under $150. It was amazing!
I'm probably forgetting some, but yeah those are the ones that stick with me the most to this day.