Doom and Doom 2, i replay them almost once a year, usually during christmas vacations. Gunning things down in doom just feels so satisfying i could keep on it for hours.
Theme hospital, its the perfect combination of casual and challenging gameplay with a cheerful style.
Plants vs zombies, played it twice and both times in one sitting, it starts easy but builds up challenge in perfect curve.
Civilization games are also really addictive, once i start i keep playing until ive beaten them atleast in second hardest AI
Wipeout HD, even when ive played this game for hunders of hours i keep getting better at it, or at least thats how it feels...
Team fortress 2. Just clocked 800 hours in that game. Its the perfect class based multiplayer shooter, though lately ive finally started to get burned out with it.