Diablo 2/D2:LoD(the game is 13 years old, I still play it often)
X-Com
WoW(even when I'm not happy with some of it, like MoP, I keep coming back to it)
Silent Hill(my favorite "survival horror" of all time)
Rainbow Six 3/RS3:Black Arrow(my favorite FPS series, and the game I spent most of my Xbox Live time playing)
1) Warcraft series - all the games considered. Love the lore and will never forget how it felt to level my first character. The world seemed so gigantic.. until i discovered there was more to it than just teldrassil
2) Unreal tournament - the very first. Set up a LAN back in the days (1998) and thanks to it, passed no courses at the U for 2 years
3) Civilization series - played the first one through the V and loved it
4) Championship Manager series - great way to turn a database and a random number generator into a game Countless hours lost to this one too.
5) Gargoyle's Quest - a little nintendo gameboy game, back in the day with black and white screen. It was truly amazing
Battlefront 2-
Ridiculous amounts of hours spent playing that game. Who doesn't remember the 'bridge of death' on the cloud city level? Sending the opposing team's forces flying off the sides. So...much...fun. You didn't even have to like Star Wars to enjoy that game. Why is there no Battlefront 3...or 4...or 5 by now? Just a travesty.
Dawn of War+Expacs-
One of the best RTS skirmish games of all time. Still on my desktop and gets played once a week or so after all these years.
Kenshi-
Open world, sandbox, RTS, with perma-death for your characters where samurais, the wild west, and sci-fi all meet.
Battlefield: Bad Company 2-
The single player was dumb, but the PvP was amazing fun. Hundreds and hundreds of hours spent in that game playing the Rush maps.
Terraria-
I just enjoy the heck out of Terraria (and I'm also fond of Darkout and Edge of Space which Terraria inspired).
1. Metal Gear Solid 3: Snake Eater
2. Metal Gear Solid 2: Sons of Liberty
3. Metal Gear Solid 4: Guns of the Patriots
4. Kingdom Hearts 1 and 2.
5. League of Legends (Does this count?)