For me, it's a toss up between the Legacy of Kain series and the Grand Theft Auto series. The first thing I played on my launch PS3 was Legacy of Kain: Blood Omen. I know, right? I get this shiny new PS3 and I spend the next ten hours playing a PSOne title.
It's a classic and it and the series have a gripping story that played a big part in what molded my gaming tastes. Crystal Dynamics and Eidos did a masterful job with that series and if we're lucky, someday we'll get more. [Nosgoth]
As for GTA, enough said.