I've become quite fond of the Shin Megami Tensei series in the last few years, having played all except one of the PS2 versions that have come out in English (Nocturne, Digital Devil Saga 1 + 2, Persona 3. Not the Action RPG one).

Also, I love love love Pokemon. Dead serious.

Those are my two favourite series. Everything else is fine, but I'd happily play a new Persona or Pokemon before any other RPG series.