I don't think there currently exist an combination that result an higher chance than 50. Where currently still buzzy to figure out how the system works but it seem that how less results how more chance you got to have one. For example when breeding Terra and Medieval together you can result:
- Elemental Metal: Metal Dragon
- Elemental Flame: Flame Dragon
- Elemental Terra: Terra Dragon
- Hybrid Flame/Metal: Medieval Dragon
- Hybrid Flame/Metal: Steampunk Dragon
- Hybrid Flame/Terra: Volcano Dragon
- Hybrid Terra/Flame: Flaming Rock Dragon
Unique Metal/Flame: King Dragon (formule&different req)
Pharaoh Dragon, Robot Dragon, are unbreedable
So in theory the best way is calculate it like this an try to find the out the list an the least results.