Leolin Sifu and Longshot definitely seem the most powerful at lvl 6.

I think it's quite interesting how certain levels can be very important for the power of a card, though. Since so far most of my CA cards are only lvl 1, for me the Abyssal Elemental feels like the strongest epic because it already has the Resurgence ability.
Others like Sifu and Longshot need to be leveled up to get some of their core abilities or stats.