For me this gen it's been Super Mario Galaxy1&2, Orange Box, and Metroid Prime Trilogy.
The creativity in both Galaxy's is amazing. The toy galaxy...woooww. And the broad scope of gameplay in #2 was fantastic.
Just a little while ago I went through Orange Box. I can understand the Half-life Hype, because it is truly a masterpiece. Portal really impressed me aswell. Such mindbending goodness.
And lastly Metroid Prime Trilogy. I hated the original gamecube controls, so much so I couldn't bear to finish the game. An updated version with massively better controls was well passed overdue. Now I can finally feel immersed in one of 6th gen's finest.