Haven't seen F4 yet, but judging by the reviews, it sounds like a turd. Out of the other three, I'd rank them like this. (WARNING, SPOILERS AHEAD).
1. Avengers: AOU - Lots of fun, action packed, and I honestly felt like the story flowed better than the previous Avengers. Some people didn't care for Ultron, but I thought he was a great villain. And the Vision was the bomb. Overall, loved the film.
2. Kingsman - This one probably would have been my top choice, but that whole Obama/not Obama head exploding thing just bothered me. I read Vaughn's reasoning for it, and perhaps as a Brit, he doesn't quite comprehend how that scene could be read here in the States. What I mean is, I can almost guarantee that theaters full of teabagger dipsh**s in rednecky parts of America erupted with cheers during that scene. I think he meant the scene as nothing more than a joke, but I can see all of the "DURR OBUMMER AINT MUH PRESIDENT" types thinking he was throwing them a bone.
3. Ant-Man - This was a solid film, and most of the performances were good. Yet something here just seemed missing to me. It's hard to say what it is. It just felt like Marvel was phoning it in a bit here. The villain plot almost felt nonexistent here, especially when they threw Hydra in at the end. The whole time, Hank is like "PEOPLE ARE GOING TO DIE!" and it's not really clear why he feels that way, and then it's like DUH HYDRA at the end. I don't know. It was a good movie, but it feels more forgettable than what Marvel typically puts out (still better than Thor 2 of course).