Wow, this is tough. Ok, here goes:
1. Super Mario Galaxy
2. Uncharted 2
3. Wipeout HD
4. Pokemon Diamond/Pearl/Platinum etc
5. New Super Mario Bros. Wii
6. Gran Turismo 5
7. Burnout Paradise
8. Okami (Wii)
9. Grand Theft Auto IV
10. Guitar Hero 3
I know most people have forgotten about GH3 - about the whole series, really. But it was immense fun when it came out, and for the fact that I put 100 hours into it on the Wii version a couple of years ago, and have great memories of it (even though when I play it now, it's not great) means I should put it on this list. It was awesome at the time.
Tough, tough list. Had to leave out Uncharted 1, Super Mario Galaxy 2 and Katamari Forever, not to mention Ratchet: A Crack in Time and Rock Band 2. Argh!