In no particular order:
Red Dead Redemption:
Probably the best (or one of them) incapsulation of videogames as immersion devices, the level of detail in the world building is unmatched and the gameplay can get seriously engaging and fun.
Its flaws are mainly dictated by R*'s stubborn unwillingness to abandon the dated mission-based gamedesign from GTA, to approach a more freeform style that would benefit their open world games.
The ability to be a rancher, a sheriff, a gang leader would've made the game that much better, instead of having to follow the linear and bland missions (albeit sustained by a great story).
Regardless, one of the most unique and intense experiences i've had this gen.
Skate 2:
Some things about the first one were better, like the more realistic and natural city design, but in general i regard this as the peak of the skate simulation genre in videogames.
The physics based gameplay and near perfect animations blew my mind in the first game, and Skate2 tweaked that "tech demo" in a more complete package.
Dark Souls:
Although Demon's Souls has also its strengths, i think Dark Souls is incredibly more meaty and rich, and i love the open world, labyrinthic level design.
The combat, the monsters and world design and the general rich and deep lore and gameplay, made this a very rewarding experience.
There's just so much stuff to discover and do, it's incredible how deep into the rabbit hole you can dig, and still discover new stuff.
The forboding atmosphere is also incredibly pulled off, though in this aspect, Demon's Souls was probably superior.
Honorable mentions:
SIREN New Translation/Blood Curse:
This game, alone, justified my purchase of a Ps3 in 2008.
One of my favorites survival horrors, with an amazing visual and sound design, and an incredible atmosphere.
They improved the controls from the previous games, but also made it a tad too easy once you get your hands on a decent weapon, which is the only flaw i'm willing to move to it.
Machinarium:
I'm not a big adventure games buff, but this game was so charming that i had to play it.
Turns out is one of my favorites from this gen, beautiful art, challenging and fun puzzles and the best soundtrack in a game, ever (yes, like in "of all time").
Listen to this shit:
Mr. Handegote.
The Furnace.
The Bottom.
Makes me watery eyed.