For over years I've been coming back again and again to Civilization I, SimCity 2000 and Castle Wolfenstein (there's something about shooting nazis I think).
In the last couple years it's two types of games:
Civ 4, SimCity 3000 and Freelancer because I have to.
Fallout 2 and Ultima 7 because I never quite finished either (ouch).

Ditto for the Baldur's Gate 2 love-in (have to confess, I prefer parts of BG1, but the game is hella difficult when you stray from the beaten track.)
Would also like to add that I have only completed Ultima 7 once, 7pt2 once and 8 once, yet I have played U4-U8 more times than I care to remember.
Also, gotta love the Sierra "Quest" series, specifically (but in no particular order) SQ3, SQ4, KQ3, KQ5, and QfG4.
Oh and I also go all-in for Fallout 2 and UFO: enemy unknown, even tho again, I only completed them once, despite many hours playing.

Well iv been coming back to....
Little Big Adventure 2: As I just love this game, it’s so fun and it works unlike LBA (Cries never got to play it)
Doom 2: Love the music, Love the Monsters, Love the Double Barrel Shot Gun and Love the Game. I <3 Doom 2
Dungeon Keeper: It’s good to be bad; it’s also good to slap things as well like chickens

Civ 4
Sim City
Command & Conquer: Zero Hour
Super Mario World
Yoshi's Island
Mario Kart 64
Zelda: Ages & Seasons
Advanced Wars
I don't know what it is, but I play these games over and over again. They are the Best!

Ahhh the 2 Fallouts....those were the games that gave me an appreciation for the oldies such as the Inkspots and Louis Armstrong. (Failout 3 doesn't count damnit...it is an abomination cobbled together from the corpses of the first two games and then resurrected by the dark necromantic arts and made to hobble about as if it had any semblance of life..).
Another game I go back to occasionally is Freelancer. A very fun (in my opinion at least) space/trading sim that doesn't require a damned joystick to operate.

I'm surprised no one has mentioned Duke Nukem 3D yet. Some of the flawed level designs and arbitrary puzzles irk me to no end, but the game mechanics and art style are just so perfect. I've only managed to fully complete the "L.A. Meltdown" and "The Birth" episodes, but every couple of years or so I reinstall the game and blast my way through the L.A. levels again.
Ditto for the Doom series, even the harshly criticized Doom 3.

Commander Keen & Jazz Jackrabbit

Resident Evil 2, Silent Hill 2 and the first two Broken Sword games... oh, and Parasite Eve :)

regarding JudasIscariot
occasionaly I go back to many games of my schoolage, but those two series are games everytime near my hand.
But other games I install occasionaly... JA2, lineage 2 (i really dont like WoW) and all those good RPG games (mainly NWN, reminds me that I should intall it again, its one of "pile of shame" games cause i havent finished datadiscs yet)
and one point for "failout 3" best name I've seen so far

1st of all Lucasarts classic adventures like DoTT and S&M. Then - and i know that i am the only one on the whole wide world :D - Trespasser. And on the 3rd place Grim Fandango or HL2. I am not sure which i played through more.
And i tried Oblivion the most for sure. Without OOO it's plain silly, with OOO it's too hard. Hard to plz me. :D

Well, maybe the next time i'll take your advise. ;-)

I completely forgot to mention roller coaster tycoon
and all its glorious ways to murder people :)
and when you hear a coughing in the distance and look around and find puke all over the place...
that's it, I'm reinstalling it right goddamned now

For some odd reason I can't quite get my head round, I keep going back to Sports Car GT - I think the difficulty level must be pitched at 'just below idiot' , the way I like it.
And - proper showing my age here - I'm pretty sure I have a ZX Spectrum emulator and a stack of old games lurking around on my hard drive....old Skool ! (daze)....

Shining Force II/Final Fantasy VIII. :)