Gears of War 2 Horde Mode.
It changed the entire co-op experience and influenced dozens of other games to include it. Today, it is almost obligatory for every game to have their own type of Horde.
And definitely one of my funnest gaming experiences.

LOL. Sorry, it's so difficult to explain and some people wouldn't even consider it revolutionary. I'd have to sit down and play through a couple of hours of the game, pausing every minute to point something else out that may have gone overlooked. It's just so frigin' clever and I've played it 11 times now. Just subtle things that no other game has ever even attempted or even considered. Making people human is a diffult thing to do at the best of times and it has nothing to do with big set pieces or earth shattering dialogue. It has more to do with a slight twist of the lip when a character says 'I've seen the merchandise and it's a lot', or an awkward laugh when someone tells the main character they are infected instead what you expect, or a glance at a watch, or a push to emphasise a sentence and give it more weight without adding unecessary words, or ONE word meaning more than any amount of dialogue in the entire 13 hour playthrough 'OK'., or an off the cuff cutting remark like 'Your watch is broken' in which we FEEL what Joel feels because we experienced the loss through gameplay.   ... I could go on and on.

So much of it is taken for granted.

Still one of my fondest online gaming memories.  I wonder if there's still much of an online community.  I should fire it up for old time's sake.

Just finished it a few nights ago.  ALMOST made me cry when winter came.... it oozes quality.  Don't think I'll be doing another playthrough any time soon, though.  It felt like one of those "experiences" that only come around a few times every gen.  Glad I avoided all spoilers.

Gotta do the Left Behind portion and then toss it on the shelf.  Excellent game.

Thought you might enjoy Hitman because of the various ways you can handle any confrontation and the creativity you could use.  Some parts of TLoU made me feel like that.  It's cool if it isn't your cup of tea, though.

1. Halo:CE ( A game which helped to stop the huge tide of second world war shooters and for that I am gratefull.)

That's practically the only one though some notable mentions include:
1. Ratchet and Clank ( fps style + RPG + Platformer )
2. Uncharted ( Helped to bring in games of the cinematography type, which lead the way to TLOU.

See my edit for why TLOU felt different for me. This feeds into why I enjoyed the gameplay more too because there is a genuine connection between the narrative and the events. It's a true journey and not just a fake backdrop for gameplay.

Halo was pretty big not only for console FPSes but for the Xbox as a whole, I don't know where the brand would be today if the original game did not launch with Halo CE.

I would say GTA III was also pretty big for sandbox games.

Yeah, if you're going to recommend a hitman game to someone you recommend any of the other hitman games besides absolution--even the mobile game, hell, even the sniper mini-game that came as a preorder bonus for absolution.

Halo helped put Xbox on the map and also started the line of more FPS games coming to consoles and helped popularize online multiplayer on consoles since most games at the time didn't feature much of it.

RE4 had the Mercenary mode which was pretty similar.  It was only single player though.

Star Wars The Clone Wars had a multiplayer mode which was basically horde mode:

"The Academy training game is all about survival. One to four players try to survive as many waves of enemies as possible. Each wave, the number of enemies and their type changes. Every third round is either a duel or a bonus round (in the Thule Moon version). There are only two locations for this game type and they are both different. In the Geonosis version you try to survive waves of Battle Droids, Super Battle Droids, Dwarf Spider Droids and Geonosian warriors while playing as a Jedi Knight or another character. The Thule Academy version has you locked in an All Terrain Experimental Transport or a TX-130 Saber-class fighter tank fighting waves of NR-N99 Persuader-class droid enforcers, Single Trooper Aerial Platforms, Ground Armored Tanks, Heavy Artillery Guns, IG-227 Hailfire-class droid tanks, OG-9 homing spider droids, Armored Assault Tanks, Dark Acolytes, and Protodekas."

http://starwars.wikia.com/wiki/Star_Wars:_The_Clone_Wars_(video_game)

I played the crap out of that back in the day with my brother.