What's the oldest game that you can go back and play and enjoy?

Mine would have to be, that's right you guessed, either Ocarina of Time, Mario 64 or Metal Gear Solid. I love these games and playing them bring back my childhood memories :)  
 
So what would your games be?

Now that I think about it, probably something like Donkey Kong. Can still enjoy that, for sure.
Wouldn't be surprised if people said Pong or something, I just wasn't a part of that generation when that game was so big.

Pong, Frogger, Pacman, Tetris.  
 
FrogSox

My oldest games are Beyond Good & Evil and PoP: Sands of Time and I do enjoy them when I play them once every year.

Pac-man, Super Mario Bros 3, Halo 1, Fable 1.
 
dont know if the ones on the original Xbox count >_>
 

I still play River Raid and Pac-man etc from time to time.

C&C, Red Alert.
 
or failing that, Warcraft 2. I've got a soft spot for Super Mario Bros 3 as well.

Contra 3, street fighter 2 turbo

Besides the obvious (pong,pacman,tetris and early console games) my favourite generation of games was the golden age of PC gaming in the 90s. I play games like Ultima,System Shock,Privateer and infinity engine games more than I play modern console stuff,which I still find pretty shallow compared to the classics.

my favorite old game would easily be Conkers Bad Fur Day. however the oldest game i have DooM from XBL. i also have the original Half Life on steam and recently i got battlefield 2 for the pc, soon after BF BC 2 came out.

definitely Super Mario Bros.

For sheer age Super Mario Bros hands down. I also enjoy revisiting Fallout (almost religiously), Silent Hill/Resident Evil, Starcraft, Diablo and a few other choice games from my youth.

chess. 
 
Seriously though, Asteroids. I can play it for hours.

" For sheer age Super Mario Bros hands down. I also enjoy revisiting Fallout (almost religiously), Silent Hill/Resident Evil, Starcraft, Diablo and a few other choice games from my youth. "

super mario bros easily

I play Unreal and Chrono Trigger about once a year. Right now I am playing through Terranigma. I just finished replaying Lufia II, and it holds up better than most old school RPG's. As hard as I try though, I can't go back to NES games, even though I grew up with that system.

Super Mario Bros, Super Mario World, Chrono Trigger, Final Fantasy 4, Metal Gear Solid, Ocarina of Time... I just like old games.

I have no time for anything but the current gen. I leave the past behind. 
 
That said, I think the current generation will hold up for a really long time, longer than anything before it. I don't even want to look at nasty ass PS2 games anymore.  
 
 
I did play Final Fantasy XII for 40 hours and Persona 3 FES for 20 hours earlier this year though. But those games are great and hold up really well. I would go so far as to say they're current gen games.

DOOM, Rainbow Six, Duke Nukem 3D, Quake. Not that old, I know, but they were my younger years.

Pong, Space invaders and Asteroid are the oldest games that I still enjoy.

I can occasionally enjoy some Pitfall, Pac-man and Asteroids.

Pong

Dungeon Keeper and Heroes of Might and Magic 3, among other games.

Pong, Tetris,  Pac-Man, Space Invaders, River Raid and Pitfall.

Probably X-Com: Ufo Defense. Sadly I've lost all my old consoles over the years, but can still enjoy Chrono Trigger and Zelda: A Link to the Past every now and then on an emulator.

Warcraft 3

Either Super Mario Bros. 3, or The Legend of Zelda: A Link to the Past.

starcraft 

Spyro maybe?

The Secret of Monkey Island. 
 
I'd be quite happy to play Super Mario Land again if I still had my gameboy.

I can still play the classics from the NES (SMB, Metroid, Castlevania, Kid Icarus, Ice Hockey, etc.)

A lot of those old arcade games and 2600 games are still fun.  Hell, pong is still fun.  I played Yars Revenge for a few hours earlier in the year.  
 
The LEAST playable games for me are the first 3D poliginal games.  Of those PS1 era games it's only the cream of the crop that enjoyable (Mario 64, Metal Gear Solid etc...).

Punch out for NES

One of my favorites, which I doubt are the oldest I still play, are Fallout 1 and 2.  They aren't the oldest like I said, but they haven't aged all that great. 

Super Mario Bros. for the NES.

Akalabeth: World Of Doom, aka Ultima 0

I can play plenty of games in theory (OoT, Tetris, Space Invaders etc.), but the old games I play regularly are Age of Empires II, Roller Coaster Tycoon and Harvest Moon: A Wonderful Life. I've played a whole bunch of Harvest Moon this very weekend.

Playing FF VII right now and I'm going to pick up VIII on PSN. 
 
FrogSox

" I have no time for anything but the current gen. I leave the past behind.  That said, I think the current generation will hold up for a really long time, longer than anything before it. I don't even want to look at nasty ass PS2 games anymore."

Delta force: Land Warrior.... I love that game...  

Bar things like Pacman, Space Invaders (which are freaking sweet) the oldest games I can remember actively playing is Manic Miner and Jet Set Willy. Manic Miner came out in 1983 and Jet Set Willy, a sequel, came out in 1984.

EDIT: Though it should be noted I didnt play them at the time, it would have been quite a feat as I was born in 1990. I played them when I was about 5 and still go back and play them fairly often.

mgs1 is a classic, enjoy it everytime i play it again

Kirby's Adventure, Super Mario Bros. and Sonic 2. I bust out my NES and Genesis every once in awhile for the sheer joy of playing them.
 
Oh the fond memories... I clearly remember playing Kirby's Adventure and Super Mario Bros. at a laundry mat when I was a child. I would sit under that table in the cubbie hole playing while my mom did all the laundry. Whoever came up with that concept was a genius!