It's a tough one between 2008 and 2010. I'm from the U.K, so I'm counting European release dates. I'm also not counting any ports/remakes.
2008 saw:
Super Smash Bros Brawl
Mario Kart Wii
The World Ends With You
Metal Gear Solid 4
Little Big Planet
Final Fantasy Tactics A2
2010 saw:
Super Mario Galaxy 2
Rune Factory Frontier
Final Fantasy XIII
Dragon Quest IX
Donkey Kong Country Returns
Bioshock 2
Tatsunoko vs Capcom
It's worth noting however, that most of 2008's great games were concentrated in the first half of the year, by the end of June all the games I listed, with the exceptions of LBP and FFTA2 had been released. 2010's were far more spread out. Early 2008 was IMO easily better than any other similar length period in gaming, but as a year it is very hard to choose.
