7th gen:
Call of Duty Black ops: thought it'd be a fun shooter...i spent 36 days on it, I love my L96A1.
Super Mario Strikers Charged: Incredibly hectic and fun.
Twilight Princess: A lot bigger than I imagined and I enjoyed aiming with the wii remote
Batman Arkham City: The combat is addicting.

8th Gen:
Assassin's creed 4(Wii U): Fighting other ships was a lot of fun. The sailing reminded me of wind waker.
Pikmin 3: Beautiful, fun, and overall one of my favorite games of all time. You get emotionally attacked to the Pikmin.
Donkey Kong Country: Tropical freeze: I 200% this game. It has tight controls, great visuals, and is challenging,  rewarding, and very fun.
Super Mario 3D world: wasn't intereted in it til the direct before it launched. I played it with cousines and my girlfriend, had lots of fun with it. 

God Of War III: I expected a lot from this game after playing through the other 4 games, but it still blew me away, much more than I expected. There are entire sections of the game I still remember, and I can never forget those boss battles against Poseidon and Cronos

GTA V: Again I expected to have quite a bit of fun with this game, but it still blew me away by the sheer scale of its world. It was almost unbelievable to see that huge mountain range knowing that all of it is explorable. And driving down from the cliff of Mt Chiliad from one of its slopes in an Adder was just exhilarating fun. The heists were also amazing.

Bayonetta - Went in expecting nothing and found the best hack n slash ever.
Vanquish - Never even heard of it until years after it's release. Insanely fun TPS gameplay, puts Gears to shame.
Bioshock 2 - No one expected from this game coming from the first Bioshock but I actually liked this game far more. The gameplay shits on the first one, and even Infinite too. The weapons, the plasmids, the traps, the bot minions. This game was just a joy to play.

I'd say Majin and the Forsaken Kingdom and Dragon's Dogma, the later being one of my favourite games, ever.

EDIT: also the modern Resident Evil games with 5, 6 and Revelations. All great games.