Halo Reach
Mass Effect 1& 2
Jurassic Park Operation Genesis
Hitman Blood Money
Assassins's Creed 2
Freedom Fighters
Uncharted
Metal Gear Solid 2
These are my personal favorites off the top of my head. To me, and I'm not very versed in music so take it for what you will, what makes a soundtrack so memorable is theme. A lot of games have great music, but they lack a distinctive theme that gives the score an identity. Blood Money has a couple of motifs that re-appear every so often, but, even though a prominent theme is missing, it's still an amazing score. That's my exception. The rest of these scores have a familiar theme, a recongizable piece that's distinct. If a game soundtrack has a good, memorable theme, then I like it that much more.