For me it's the Super Smash Brothers which is available also in PS3. It's a very enjoyable game where you could be any Nintendo game character you like. Another good one is the old Capcom vs Marvel game, where still playing it now.
My favorite PS3 game is The Last of Us. Back when it came out, I was so engaged in the gameplay that I thought to myself, this is the best game I have ever played. The graphics, the story and the mechanics were top notch. I remember watching Pewdiepie's playthrough of this game. It was a memorable one and the comment section wasn't as toxic as now. Everyone genuinely enjoyed it.
This might be seen as a weird choice, but LittleBigPlanet 2 is probably my favorite PS3 game. I like the whole vibe about this game, because it's really colorful and it requires creativity. There are an infinite amount of levels you can play, which different people over the world have created, which is amazing if you ask me. It's the perfect game to relax in after a long day of work.