First of all, a bit of a disclaimer: the value of games rises and falls with fashion. At the moment, retro is quite in, so prices are high, despite the fact that virtual consoles and emulation bundles are all over the place. Some people still want to have a shelf full of physical things, so much so that a few games in the loft at your mum's house may be worth thousands of pounds. Perhaps you didn't waste your teenage years after all.
1. Nintendo World Championship, NES
This is nothing to do with fashion -- Nintendo made little more than 100 copies of this NES compilation to hand out as prizes at the start of the 1990s, hence there are hardly any in circulation. Ones that do go on sale fetch tens of thousands of pounds, and if you've got one of the gold ones... you could conceivably make enough from selling it to put down a deposit on a house.
2. Air Raid, Atari 2600
A handful of copies of this have been sold in recent years, with a boxed example fetching around £25,000. Unboxed it still fetches thousands, so keep an eye out for one at any car boot sales you might attend. You'd get on the local news if you bought one off an old man for 50p.
3. Red Sea Crossing, Atari 2600
This religion-themed Atari game didn't exactly go sales mad, hence only 100 cartridges are thought to have been produced. Auctions place its value at around £6,000 should there be on in a box of forgotten things in a cupboard somewhere.
4. Darxide, Sega 32X
Switching to games we might conceivably stand a chance of owning in the UK rather than US pipe dreams, is this. Darxide is a really quite terrible-looking 3D shooting game from the back end of Sega's console glory period, which virtually no one bought when it launched in the UK. That, of course, is good news if you were suckered in to Sega's 3D gaming attachment and picked it up, as a decent copy in a nice cardboard case -- practically impossible as the boxes were terrible and usually fell apart on the way home from the shops -- could fetch well over a grand.
5. Primal Rage, 32X
The slightly dubious list of valuable games assembled by Love Antiques has also broken its data (which we disagree with much of, but still) down by console, placing the 32X version of Primal Rage just behind Darxide with a projected value of around £1,000. Some of you must have a 32X in the loft, surely? The Atari Jaguar version -- on CD! -- is worth a bundle, too. Which must mean no one bought it back in the day.
6. King of Fighters 2000, Neo Geo
The amazing arcade machine format was home to some right old classics, including the bulk of the KoF series. It was for rich kids only back in the day, and still is today -- with cartridge production numbers in the hundreds and many games ruined through being in arcades 24/7, there aren't many about. A copy of this that works ought to fetch around £4,000, if you can find someone of the right age and wealth demographic to agree to take it off your hands.
7. Snowboard Kids 2, N64
Someone recently paid £175 for an unboxed PAL copy of this, so either the buyer's a sucker or that's what the internet has decided it's worth. A boxed one would obviously go for many multiples more. You were probably a Mega Drive kid, though. Always missing out.
8. Fatal Fury Special Edition, Sega Mega CD
All those years of being the Mega Drive kid are really paying off now, though, especially if you had enough money for a Sega CD and a thirst for import games. This ought to fetch around £400 if you've still got the box and manual.
9. Shining Force III, Sega Saturn
And then, if you were still in to Sega enough to buy a Saturn, there are some valuable things out there too. Highly though of RPG Shining Force III ought to be worth well over £100 today, should it have been sympathetically stored in a damp-free environment for the last 15 years, and if the you of 1998 didn't ever use the manual as a coaster.
10. Power Drive, Sega Game Gear
Recently sold for £146, and that was with a creased manual. Why on earth did you have to crease the manual, mum? I always looked after things but it was you tidying up that ruined everything.