Hm, I'm not really sure how to answer this because what does definitive mean in this context? Best games by standards in terms of sales, or production quality...or games that I personally think are great?
my name starts with H, and I guess that means I'd have to do PS3.
If I had to pick 5 titles for the system in no particular order:
1. Metal Gear Solid 4 - as much as some people think the ending of this game was stupid, I loved every minute of this game. This game just did everything I wanted it to, and it was pretty awesome in every aspect.
2. Tales of Graces f - This is the only game in the series I have played to completion, and I loved my time with it. I often read on sites like this that people seem to hate the story and characters, but I loved all of that stuff. The fact the combat was great was just icing on the cake, but I thought everyone was likable in their own ways, and I liked where the story went.
3. The Last of Us - I hesitate to put this on the list for PS3, because the superior PS4 version eventually came out. I still played this through on PS3 though, and I just was so damn impressed with every aspect of this game. I don't think much more needs to be said about it, except even with it's lower graphical fidelity on PS3, it still looked amazing.
4. Demon's Souls - Not my favorite entry in this genre of game, but I can't deny the effect that this game had on me, and every other gamer out there. It felt cool discovering this Japanese game we could only import for a while, and then the genre blew up with Dark Souls. Demon's Souls was great, but it's not my favorite in the genre after all this time though.
5. Final Fantasy XIII-2 - Again, not sure if I should have this because it was multiplatform, but for me, it was an important title. Final Fantasy XIII was a major disappointment to me, and many others I think, when it came out. It had somethings right, but a lot of things wrong with it. I felt XIII-2 addressed all of that. It still had some of the baggage from the previous game, but it really did a great job of turning around my opinion on that series. Gave us a chance to actually enjoy the combat that the first game introduced, and gave us a lot more than a narrow hallway to walk down. I thought Serah and Noel were likable characters, and while the story was a bit confusing, that's kinda what I loved about it. It took some bold steps that may have been a bit much for people to digest, but once you wrap your head around it, can be pretty cool. I thought it wrapped up with an awesome end with Lightning Returns, which was also good, but not as good as XIII-2.