Dragon city has now almost 1000 dragons, and i breached the 250 mark. However, i noticed the likeliness of Counter circles in elements. Opposites or specific counters are also a lovely combo found. Take the Crested Fae Dragon. Water and ice both counter War, while Light and Earth counter Dark.
Take the Ternion dragon as Counter Circle. The 3 special elements Pure, Legendary and Primal in one. A nightmare for higher level players. All his counters are likely to be countered. dragons like Dark angel with pure as second or Burbi with legendary as third could not be countered. Poor heroic dragons.... The first breach of the 4-element dragons hit me when it landed. Suddenly dragons with 4 opposite elements arrive and could be bred. I might follow up with more

Don't worry, there are heroics with perfect coverage among their elements and those who don't have perfect coverage often make it up with sheer firepower. Take the High Resolution dragon for example. It only hits 6 different elements super effectively (Legend, Sea, Metal, Nature, War, Ice) but it has a good defensive element and some of the hardest-hitting attacks in the game.

Anyway, Heroics with perfect coverage so far are:
High Tidal Dragon (Sea, Dark, Terra, Metal)
High Fenrir Dragon (Ice, Pure, War, Electric)
High Nucleus Dragon (Pure, Metal, Terra, Nature)
High Entity Dragon (Pure, Nature, Water, Fire)
High Star Dragon (Legend, Electric, Terra, Metal)
High Purity Dragon (Pure, Ice, Metal, Dark)
High Snow Dragon (Pure, Nature, Ice, War)
Unreleased: Yggdrasil Dragon (Nature, Primal, Water, Light)

I don't really know what you are talking about when you say that Dark Angel and Burbi can't be countered, just use any old dragon that hits them super effectively, and avoids their coverage elements. However, Dark Angel is a special case since it can be trained fire and dark attacks.
The best breedable 4 element dragons so far are Apocalypse, Abyss, and Supersonic.