I offer up the "Return of the Jedi Hypothesis"
New Hope - Good, ground breaking even.
Empire Strikes Back - Considered by Many to be the best is the series, dark, moody etc.
Return of the Jedi - worst received of the original trilogy, ending the saga, failed to really build on Empire.
This pastern holds true for most Trilogies.
Back to the Future 1 - Good, not great.
Back to the Future 2 - Better than 1, builds well on events of 1.
Back to the Future 3 - decent but weak final chapter.
Alien 1 - Good or Great film depending on who you ask.
Aliens (or Alien 2) - Great film, takes the character of Ripley to a new level becomes the film most remember when the Alien Quadrilogy is mentioned.
Alien 3 - Boring, bad, forgettable.
Alien 4 - Lets not talk about it...
The same holds true for Games, IMHO. Halo, Killzone, God of War, Assassin's Creed 2 Story Arch - 2 good, Brotherhood - ok, Revelation - tired, dated, a worthy end to Ezio's story but not the greatest title in the series.
Mass Effect is a brilliant case in point. 1 good, ground breaking. 2 Best in series the Empire of the Mass Effect series. 3, decent ending to the trilogy but was never going to make it out of the shadow of 2.
It is also why, IMO, there really shouldn't be more than 3 entries into a series, film or game. Most games/films start to really suck by the 4th entry and I don't know many/any films with a number higher than 5 that is still worth my money or time. Game are a little a head in this respect, a 4th or sometimes even a 5th can still be enjoyable but the fatigue can be felt, Call of Duty is a good example of this, IMO, COD4 is one of the best in the series, 5,6,7,8,9 have all have the trademarks of a diminishing experience.