Starting with the best, Metal Gear Solid 4 has justified the purchase of the PS3 all on its own. The game is everything I expected it to be and more, and I am only about halfway through it (even after playing for twenty some hours). Everything about the game is amazing, but most importantly, it has a great STORY. My only complaint so far has been the boss battles. The two I have completed were less than epic. The characters were alright but there are much more memorable battles in the other games of the series. There is still hope though as the game continues on.
Call of Duty 4 is another title I purchased at the request of my brother. Though it is a fun game, I grow bored with it if I play too long because the game is a little too linear for me. All you do is shoot stuff. That's it. Shoot shoot shoot. Blowing shit up is only a blast for so long, then it just gets tedious. And everything in the game is pretty much spoon fed to you. I have enjoyed the story thus far and am excited to see where it leads, which is why I keep playing, to follow the story and see where it goes.
The only other game I plan on buying anytime soon is either Rock Band or Guitar Hero (leaning towards RB). People flame those games and say "why don't you learn a real instrument?" Well, a step in that direction is that those games teach kids tempo, which is very important and will be helpful if a kid does decide to learn how to play the real instrument.