The Elder Scrolls... that's about it... every other series I can think of has that one game that just didn't measure up. If we're going by the notion that the newest game is the best out of the series then there are virtually none...
God of War lost its footing with Ascension, GTA with GTA IV, Pokemon with R/S/E, Halo with Halo 4(at least their excuse is solid, new developer) Zelda with Skyward Sword(personal preference but I think its fair to say it wasn't the best entry which certainly qualifies it to be on the list), Smash with Brawl, MG with Rising(though I'm not sure thats a main entry), and Resident Evil (needs no explanation).
Now most series seem to find their footing again but the Elder Scrolls is the only one I can name as having entries that are consistently better than it's predecessors.
