Yes. That is the answer. The PS3 has so many awesome titles, that it's hard to pick one.
When I think Xbox 360 I think Halo. Maybe a few others if I spend more than 3 seconds thinking about it, but Halo is the first thing on my mind.
If there's any praise I can lob on Sony, it's the variety of games they offered and supported. Not just games like Metal Gear Solid of God of War, but they took risks with games like Heavy Rain and Little Big Planet. We might not think of them as risks now, but hindsight is 20/20 and few developers would have taken such big chances. And don't forget MAG. Games consist of as few as 64 players and as many as 256 on the same map. Yeah, you read that right.
So, I'll say that PS3 has a good variety of games to offer you. Naming one as the BIG title is hard. But that might also be a weakness for the system.