Well, Star Trek 2, 3, 4, AND 6 are heavily interconnecteded. They can all work together as one big narrative.
1, and 5 are independent jaunts. 5 only makes minor references to the bugs of the new Enterprise A. So, it can be ignored entirely.
As far as quality(my opinion):
Two is great. Probably, one of the most entertaining action/adventure films ever. Not really a Trek film, nor was it written as one. Meyer was not really a Trek fan, and Roddenberry was kicked off the production(he hated the film)....
Three was ok, largely a film to entice Nemoy to come back(who became director). But, it kinda undoes or swaps the dramatic weight of 2. It has some good dramatic moments, but the movie feels a bit cruel at times. Less Trek, more space opera. More of an extension of 2(from a different writer), than a proper indepedent entry....
4 depends on the individual. It is the most Trek film of all the OC films(outside of one). But, its a border line satire or comedy. Tons of self awareness, with actors practically breaking 'the fourth wall'. Also, it has really dated and silly politics(Nimoy) of its time. But, I enjoy the hell out of the movie...
6, with Meyer getting total production control again, isn't really a Trek film. 6 is a (Tom Clancy) political thriller masquerading as a Trek film. Its wonderful for similar reasons why 2 is wonderful. Probably the second best Trek film....