Defiinitely Dark Souls. Getting better at it and learning new strategies or trying new builds makes me want to play it over and over. It's mainly due to the way thet force you to become intimate twith the layout of each zone your first time through,

Counter Strike all version

Of course I play it with my friends

Final Fantasy VII.

Several Zelda games.

Lunar: Silver Star Story and Lunar: Eternal Blue.

Ever thought of getting into the XIII speedrunning community? They're pretty active and fun people to chill out with.

Betrayal at Krondor, the Thief series, Mount & Blade: Warband with mods, the Elder Scrolls series with mods. Mods have just completely changed how I see and play video games, I could never go back now.

Speed has never been a strength of mine.

Might give it a go though. Has been a while since I last played it!

Here, watch this (no need to watch it fully, just skip to the most interesting parts), to get a bit the idea of it:

https://www.youtube.com/watch?v=qauQSJT2jqc

Very good list!
I would add God of War 1-3, Dark Souls 1-3, Bloodborne and all GTA games. ^^

I definitely love the God of War games for replay.  The first 3 and also even Ascension I like. 

Knights of the Old Republic 1 & 2, and the Last Of Us.