Mine is ADoM, a freeware Roguelike from 1994. I've been playing it since then, too. Hugely replayabie - even after countless completions I still play it. Will I ever take a halfling merchant to victory?

Great game and there's a native Windows version, but if you try it be prepared for a brutal learning curve. Don't expect to win in the first couple of months.

I've just found out that the sequel, Jade, is back in development with a downloadable alpha release. It's only been 15 years since it was first announced :)

I play System Shock 1 every year or so. Great game, it is.

Dunno if it's the oldest, but it feels like it.

I don't really replay anything with any regularity because I've got such a huge backlog of games to play now thanks to amazing sales here and on Steam. I will say though that thanks to GoG I will be revisiting my old love, Ultima VII: The Black Gate and finally playing part 2 after so many years. :-)

I think it might be MoO.

I bust out the 2600 from time to time to play some Demon Attack and Galaxian. I actually like the 2600 version of Galaxian better than the real arcade version. Its not as pretty, but It just seems like its more fun for me to play.

Any of the Heroes of Might and Magic games, or of the Might and Magic games.

The old AD&D Gold Box games from 1988-1993.

ZZT (1991). Though, to be fair, most of the creations/mods I play are from more modern times.

Morrowind and Deus Ex are my two favorite games and I replay them pretty often. Morrowind especially is always installed on my PC so I can load it randomly.

Only one older than those I have played over and over would probably be the original Fallout.

Runescape or Deus Ex.

Goldbox d&d games, defender of the crown

I play World of Xeen sometimes. Another one I play more frequently is Daggerfall.

Doom. It gets a run through or 2 at the least each year.

Elite. ZX Spectrum. 198....4/5 ish. ^_^

I still drag out my MegaDrive for some D&D:Warriors of the Eternal Sun and Phantasy Star 4 and Buck Rogers:Countdown to Doomsday, occasionally, too.

PC - Settlers 2; C&C (w/ Covert Ops); UFO:Enemy Unknown and Terror From the Deep; Ultima Underworld 1.

Age of empires 1. I know it's not *that* old, in GOG standards, but I play it often. I'm not very good about it anyway :D But I love the game. Especially the expansion Rise of Rome.

the second one is Lords of The Realm 1.