The Godfather Trilogy
N/A - Haven't seen.
Star Wars Original Trilogy
9 - Mostly excellent, fell off a little with ROTJ.
Star Wars Prequel Trilogy
8 - The first two were lame, but ROTS mostly made up for it.
Indiana Jones Original Trilogy (Raiders, Doom, Crusade)
10 - Excellent all the way through. (If Crystal Skull were considered, the mark would be lower.)
The Lord of the Rings Trilogy
10 - Excellent all the way through, spectacular visuals.
The Terminator Trilogy (The Terminator, Terminator 2, Terminator 3)
7 - The first two rocked; the third fell way off - and draws additional docking for not being consistent with the first two.
The Bourne Trilogy
8 - Solid all the way through, but not that spectacular.
The Matrix Trilogy
6 - The first one totally rocked. The next two were a long trip downhill.
Pirates of the Carribean Trilogy (Curse of the Black Pearl, Dead Man's Chest, At World's End)
8 - Mostly good, but the story got a little weird in the third one. Also docked because of KK starving herself into a skeletal waif for the last two movies; her look left me wondering what Will still saw in her.
7 - The first one was excellent. The second one was also pretty good, even though it left one with the impression that the government must have ponied up the dough for Michaela to get implants as well as for Sam to go to college (since Megan Fox was jiggling all over the landscape in the second movie). In the third one, the storyline was lame, clinching the "7" mark -- but it did have the best FX of the trilogy.
8 - Consistently good all the way through; the third one got a little weird with the film take on the Dark Phoenix storyline.
7 - The first two were excellent; the third one fell way off. Props to Maguire and Dunst for being savvy enough to know when to call it quits, since they took a pass on making any more.
The Mummy Trilogy
Depends a little bit on what you consider to be the Mummy trilogy ...
The Scorpion King, The Mummy, and The Mummy Returns: 6, as The Scorpion King doesn't fit well at all with the other two.
The Mummy, The Mummy Returns, and The Tomb of the Dragon Emperor: 7. In this case, it's Dragon Emperor that's the misfit, but not as badly as The Scorpion King.
8 for unevenness - The first movie was excellent; the second was blah and did more raising questions than it did continuing the story. The third one, however, was excellent and resolved the issues the second movie raised.