I tend to play with deck 2 mostly because it gives me a good range with a couple of strong characters like the Giant/Baby Dragon & basic characters like archers, knights & bombers. I do like the bomb tower which can defend a large area.
Deck 3 is the strongest with all the big heavy hitters but the average elixir cost of 4.2 takes a while to recharge.
I would say anything between 3 to 3.5 is about right to have a strong team. I've level 5 at the moment & nearly have 800 trophies to unlock Arena 3.
Outside of iOS I'm still plodding along on Game of Thrones & completed Episode 3 yesterday evening. I've not yet deleted my MGS5 save so may yet go back to play Part 2.