I think this should been in the 'Movies I-X' forum. Despite the fact that they are sequels to episodes, I think "Star Trek II: The Wrath of Khan" and "Star Trek: First Contact" are two of the most accessible ones for people who aren't that familar with Star Trek and haven't seen too many episodes of any series.
I believe "Star Trek IV: The Voyage Home" is the most accessible of all, because it's funny and sweet, whether you know the characters and their histories well or not. There are some lines at the beginning and end referencing the previous movie, but not knowing what they were about didn't confuse me or negatively affect my first viewing experience. When I first saw it, I had not seen a single episode of "Star Trek" or the previous two movies, yet I loved it immediately. The ones I've mentioned are my three favourite Star Trek movies and I like "Star Trek: First Contact" most.