I'd say Final Fantasy, but it only has a few sequels. There's a sequel to FFIV, X and two for XIII, but each main installment isn't considered a sequel because they are all completely and utterly different from each other.
Breaking Bad, great show or greatest show? NeverDead: The worst game of this gen.
I can think of 2 series that at the least, needs to be put on hiatus.
Call of Duty (last game I played in the series was MW2, & WAW was the last I enjoyed playing) Final Fantasy (once the MMO was released Square tried to reinvent the wheel when all it needed was a new hubcap)
& 2 more that really shouldn't have as many sequals as they have now
Sonic (pretty much, after Sonic Adventure 2 they should have taken a break instead of releasing 1 half-***ed game after another) Assassin's Creed (did we really need to drag the story out with Brotherhood & Revelations?)