You can't go wrong with Red Dead Redemption, amazing game.
If you enjoyed UC3 definitely play the others, UC1 isn't quite as good but UC2 is by far the best in the series.
Also get the God of War Saga collection, must plays. I wouldn't recommend Ascension for the single player unless you really like the others, however it has a really fun multiplayer component so there's that.
Killzone 3 has great multiplayer, I actually enjoyed the game more than KZ2.
Resistance 3 has a great single player campaign, online features have been discontinued though.
LittleBigPlanet is a must own, if you have a PS3 go for LBP2.
List of the top PlayStation exclusives here: http://gamrconnect.vgchartz.com/thread.php?id=173867