I suppose at this point, my best deal was my Super Nintendo. I had a friend who had recently moved back in with her mom (her parents were split & she'd been living with her dad). Mom was giving her crap over owning too many videogames, so she gave me her SNES collection to appease her. It included, amongst others: Mario RPG, Zelda, Donkey Kong 1-3, Starfox, and Evo. Nothing special at the time, all fairly valuable now. I did have a couple stolen, unfortunately. Luckily it was still before the prices really took off so replacements weren't expensive. Zelda switched from cart-only to a nice, minty CIB for only $20.
Aside from that: I bought my first Sega CD with a genesis from another friend leaving for college for $20. It died shortly after. I got a replacement from a local flea market for a total of $10. It was marked $40, but they took the price down when I told them not to give me controllers or hookups except for the genesis, since theirs was a model 2 and mine was a model 1. I sold them the broken one the broken one the next week (they mostly wanted the power supply for the gen model 1). The games I got from my friend included Lunar and Vay. Later on at that same flea market, I got 2 memory carts. One in box for $5 and one loose for $1. I also got a pristine copy of Sonic CD for $5.
I had a co-worker bring me a stack of Dreamcast games since system was long gone. The most notable was Lodoss War. I didn't own a Dreamcast at the time, so I picked one up with 2 controllers for $40 at the flea market (damn do I miss that place!), and a VMU for $6. Went over to Gamestop because they were closing out DC stuff, got another VMU for $10 and Evolution 2 for $5ish. When I got home and un-taped the case, a disc fell out- they'd put a disc-only Evolution 1 in the case on top of Evo 2. I was pretty happy about that, since I'd been iffy about buying the sequel first.
I also found good stuff during the $5 Classic Plastic sale (I wish Game Crazy was still around sometimes). Tail Concerto, Vagrant Story, Zelda DX for GBC... and Suikoden 2. I wouldn't let go of that one, I just held it out for the clerk to scan.
Most of my more recent hits have been accidents. Last summer, I found some boxed SNES games for a dollar each at a yard sale and bought them, just because when am I gonna find boxed SNES games at a yard sale again. Looked up prices on Ebay later... did not realize how much Wolfenstein 3d was worth on the SNES. That was a nice surprise.