I'm trying to think of a series where I don't find the later installments less entertaining than the first, um....

Aliens was as good as Alien.

The Road Warrior was better than Mad Max.

The Empire Strikes Back was as good as Star Wars.

Goldfinger was better than both Doctor No and From Russia With Love.

Stormbringer was better than The Dreaming City.

The Tower of the Elephant was better than The Phoenix in the Sword.

The subsequent seasons of Star Trek: The Next Generation were better than the first.

The last season of Enterprise was the best.

The Terry Pratchett novels without Rincewind (and especially Two Flower) were better.

And Rocket to Russia was a better album than Ramones, and Leave Home,

otherwise I can't think of any works that don't lose something compared to the earlier installments.

Change is usually bad for me, other people find things improving?

A thread saying what series get better interests me!