For almost all film franchises it's prts 1 & 2 that are best

Beside Christmas Vacation (1989), Rocky III (1982) and IV (1985)
I guess people might find it easy to live with out most of the following third and fourth chapter titles, where as for me, I really enjoyed most of them. Some in fact, I love way more than the rest of the franchises which they are a part of.

Halloween 3: Season of the Witch (1982)
Jaws 3-D (1982)
Death Wish 3 (1985)
Police Academy 3 and 4
Howling 3: The Marsupials (1987)
Speedzone aka Cannonball Fever aka Cannonball Run 3 (1989)
Critters 3 (1991)
Ghoulies 3: Ghoulies Goes to College (1991)
Maniac Cop 3 (1993)
Leprechaun 3 (1995)

This is true for many, but not almost all.

Wrong.

Though there are exceptions, but this is generally true.

As a typical franchise goes on and more sequels are made, it gets harder to beat formula and repetition.

I think that much of the problem is that movies aren't intended to become series to begin with. If they were written to become series then they would turn out better. A lot of it is due to money concerns. There's a Chinese animated series of films, the Kuiba series, that while it wasn't the best series ever, managed to stay pretty consistent in quality until they ran out of money because their films didn't catch on with the Chinese public. For better or worse, quality work doesn't always make a lot of money. The good news is though, that new tools will let filmmakers eventually create films with greater ease than before. First regular animated films and then pseudo-live action films as the technology improves. It should make it possible for a film to hit less snares in the production process and thus allow for longer series than were common before.

I prefer Toy Story 4 & 3 to 1 & 2.

Come at me bru.

Agree for most films

My exceptions would be:

Die Hard (part 3 is superior to 2)
Indiana Jones (last crusade is much better than TOD)

I have to agree with Rocky though, part 1 and 2 can almost be viewed as 1 film. Both are so superior to the sequels that followed. I know people love IV but it’s so over the top it’s silly. Rocky wins the Cold War and has a robot.

Nova just proved you wrong with the living dead series. Like most series, it's the odd number ones that are inspiring and the even numbered ones that are Hollywood contrived formula movies. 

NIGHT OF THE LIVING DEAD, then the contrived, predictable failure of Dawn, then back to a more original DAY OF THE DEAD

The classic JAWS, followed by the retarded Jaws 2 which we have to admit is one of the 100 worst movies ever made, back to a watchable JAWS 3 then down to retard level again with #4

Classic STAR WARS A NEW HOPE, followed by Empire, which doesn't even have an ending and just does a lot of mindless action for the geek squad, back to a real story in RETURN OF THE JEDI

Even the Star Wars series as they came out, #4-6 came out first to stun us with greatness, then 1-3 was a total groan effect of poor writing, back to classic style superior scripts, story, and characters in 7-9. Whatever else, even the haters have to admit Kylo was the most credibly motivated and interesting of the Star War villains (if the haters are honest).

STAR TREK flip flopped the first two, with the first some garbled mess, and the second a good film, but after those, we see #3 and #5 undeniably being the two best of the entire series. Certainly the best in writing and character motivation.

The only exception is ALIEN, whose better movies were even numbered.