The answer is easy for me: Final Fantasy, I grew with them, and they always tend to be different one from the other (unlike some other series games), and we have a lot of'em! Plus they really have looooong gameplay time... I may not agree about the replay value fact. Once you've played them, the story turns repeatitive unless you are replaying to find all the secrets, or maybe you haven't played it for a long time (I mean, it was amazing to find that Cloud was that soldier rescuing Tifa when they were younger, but seeing it again wont cause the same effect... O.o)
In any case, I'm a big fan of the FF series, so if I were to play only one game series for the rest of my life, I'd pick FF.