Generally, the even-numbered movies have been better than the odd numbers. I enjoyed them all, but generally that's the way of things.

Except for X. That was disappointingly weak... which is what the shows have become in recent years.

I have no idea if this answers the original question, since I am unable to make sense of
wich movie is their klingon for a good time
that would be worth presenting to the class?
Oh, wait, you mean you're looking for a movie where the Klingon language is spoken for a long time?
VI is good. There are also many episodes of Deep Space Nine that focussed on Klingon language and culture. The first episode of series four is a good one.

I definitely can't recommend showing an entire film or episode to your class, though. Selecting a few lines of dialogue, a few conversations should be enough.