FOLLOWING INFORMATIONS COME FROM DarkFrozenDephts youtube channel.
Link to channel : www(dot)youtube(dot)com(slash)user/DarkFrozenDepths

Light Enemies (best for gems)
Pearl- 650 Gems
Diamond- 700 Gems
Holy- 750 Gems

Dark Enemies (best for rank points and Exp)
Sapphire- 400 RP and 500 Exp
Dark- 800 RP and 1000 Exp

Emerald dark ennemy doesn't give much of any...