Fast 5 & 6 are easily the best of the series. I would say check out the other films too just so you get the idea of the story and some of the little jokes in 6.
I haven't seen Star Trek yet, but I'll probably check it out this weekend. From what people tell me you either love it or hate it, but I like the series, and the idea of the reboot so I'll probably fall into the love it camp.
I agree about Iron Man 3. There was a lot of fun, but it didn't feel like a full movie.