Mulligans
General Mulligans
In general, Town Crier is the best one drop in the deck, it draws so many good cards. Eternium is also good in general as a 1/3 in stats alone is pretty good. Chopper is just a solid invoke card.
Aggro Mulligans
All of these choices either gain you armor, do AOE damage, or can combine to help with tempo.
Control Mulligans
Not to keep in the Mulligan, but just a general combo against control. It can do 16 damage to face.