I've already tried Dishonored, AC3, Arkham City...I grew tired of AC 
Dead Rising 2, Portal 2 deserve a shot i guess...
How big is the Walking Dead to download btw? And Garry's mod has local multiplayer o.o?
And...I have Dead Island: Riptide
Walking dead is not too big, But to understand what've leaded to Walking dead 2. I recommend you downloading all the episodes of Season 1.
Season 1 is OVER 2 Gigabytes.
However, Season 2 Episode 1 was 600+ MB As I remember.
But it got the story necessary that made it a successful series of games.
And Garry's mod Does not includes Local Multiplayer.
However, You can play it using the localgame mode.
Just for others computers that uses your Network.
All you can find in Garry's mod is FUN and creative. (Me and 2 of my friends once made a spaceship.)
All you need is a bit of knowledge to make some great stuff in Garry's Mod 13.
By the way, Do NOT update it. I warn you! (Garry added anti-pirate to the game, So, DO NOT UPDATE)
Dead Rising 2, You will really like it. I'm serious, You won't stop playing it till you are done. Same for Portal 2.
Assassin's Creed IV: Black Flag is different than the others. (As I'm told.)
And about Assassin's Creed 2, I won't argue about it. Play it or lose a lot.
More Suggestions:
(Adventure): L.A. Noire.
(Adventure/Mind challenge): Bully: Scholarship Editon. (It contains mind challenging mini-games.)
(Create or Play Created Modification GTA SA): Design Your Own Mission(DYOM). (It doesn't require CLEO, But to save missions you have made, It requires CLEO) (Nothing to do with multiplayer)
Have Fun.
