With my N64 and Genesis, one cartridge will end up staying permanently in the slot of the console. It ends up being my favorite game of the console.

In my N64, Mario Kart 64 has stayed in the slot for 15+ years. Me and one of my best friends hook up the old system and will play battle mode every couple of years. It's like being a carefree high school student all over again. Fantastic game!

With my Sega Genesis, NHL 95 has stayed in the slot for 20+ years now. One of my favorite games of all time! I'm thinking about getting NHL 94, 96 and 97 simply because they are now so cheap (a game exchange had them for $2 each). I've also never owned an XBox, but may consider getting NHL 10 ($5 at GameStop) and the console (once the price drops down to $100 when the 720 is out). Quality NHL games seem to be woefully lacking on the Wii.

I've looked into purchasing an old NES, SNES and/or a Retron 3, but most of the games I would want for the NES or SNES I have already purchased on the VC (minus Bionic Commando, Contra [what's up with that!] and the Donkey Kong Country series [taken off the VC without warning before I could purchase]). Although it is fun to purchase and play old games, I'm really only interested in games that I played as a kid and have good replay value. Plus, I only had the Genesis and never owned a SNES. That being said, I only played the Mega Man series a handful of times as a kid, but now have every Mega Man game on the VC!

What one game ends up in the slots of your old consoles?

None. When I am done playing the console the game comes out and goes back on the shelf with the others.

I usually have Zelda in mine. I like to pick it up and play some MQ from time to time.

Whatever game I was last playing stays in the slot, until I get a new game and end up leaving that one in instead. For instance, the last 3DS game I played was Mario Kart 7 a few weeks back, and it's stayed firmly in the game slot on the 3DS ever since regardless of whether I actually play the game.

As for what game stays in the game slot on an older console... probably whatever one I was playing before I got a new system.

Kid Icarus for my 3DS, No More Heroes 2 for the Wii and I always take out the disc in the WiiU.

Harvest Moon and/or Pokemon.

Ok yeah I have to admit my handheld consoles I do have a game in the slot since thats the best way to keep dust out (except for the DSlite which I just use the little cover that came with it for that purpose). But the OP said old consoles so I didn't include my 3DS which yeah has MK7 in it from the last NL community gathering.

My DS though has Dawn of Sorrow in the DS slot and Zelda Four Swords for GBA in the GBA slot.

My GBA SP has Super Mario Land 2 in the slot.

Monster Hunter Tri has been enjoying its stay in my Wii U the past few weeks. And Fire Emblem Awakening has been in my 3DS.

EOIV has found permanent residence in my cartridge slot.

I don't keep just one game in the slot, it's often what I was playing last