When 1.0.8 was released, I suspected that Epic would be the hardest to raise to Mythic (see Mythic troops count), but now I have confirmation.
Out of 158 troops:
I have 4 troops at epic level, all are originally epics. On average, I need 12 cards of each to raise them to Mythics.
I have 41 troops at legendary level:
- 11 are originally legendaries. Including the 2 imps. On average I need 2 cards of each to raise them to Mythics
- 30 are originally epics. On average, I need 5 cards of each to raise them to Mythics.
The rest are at Mythic level.
Therefore, I still need about 200 epic cards and 20 legendary ones… Considering that you get ~5 epics to each legendaries and that there are a lot more epics, it makes legendaries the hardest troop to raise to Mythic levels.