Moria and it's descendant,
Angband. It's been a freeware development for over 15 years. I started palying it in 1989. I've never solved it. It's a "graphical" dungeon, using ASCII characters to represent yourself (@), monsters (A = ant, y=yeek, j=jelly, o=orc...), townspeople (t).... etc.
Angband is based on Tolkein fiction, so you can choose to be a Hobbit, Elf, Half-Elf, Human, Dwarf, Half-Orc, Half-Troll, Dunadain (Westernesse), High Elf, Kobold or a Gnome. You can choose as a class (i.e. job) Warrior, Priest, Mage, Paladin, Ranger or Rogue.
The stats (Strength, Dexterity, Constitution, Wisdom, Intelligence and Charisma) are basically the same as AD&D with 18/100 being the highest possible.
There are MANY variants on Angband, each with a different theme. For example, ZANGBAND is based on Zelazney's Amber series instead of Tolkein.
I always return to this game. When I get tired of it, I stop playing, but it's always on my hard drive, or two clicks away from downloading the latest version. It will fit on a floppy disk to this day.
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As for modern games, I played the original Tribes for a long, long time. For a time, I played several hours a day. And I played at least on the weekends for, I don't know, at least a year and a half. I was mostly into the shifter_v1 mod. I was very active on alt.games.starsiege.tribes during that time.
At the moment, my addictions are GTAIII, and Diablo II: Lord of Destruction. I have plenty of other games, but those are the ones that I consider addictive.