My favourite PS3 game is Yakuza 4. At heart it's an RPG with surprisingly deep action-based combat as its primary component, but it's a game so huge and with so many fun distractions that you're bound to find plenty more worth doing.
Also highly recommended:
Valkyria Chronicles (innovative and beautiful strategy RPG)
Sly Collection (three accessible PS2 platformers tarted up in HD)
Uncharted 1 & 2 (spectacular cinematic action games)
Jambo said:
Bayonetta for some action based good times.
Bayonetta is brilliant but seeing as you have access to both HD consoles I'd recommend the 360 version. The game was developed for the 360 then apparently given a quick and dirty PS3 port, and the results were
not good.