Monster Value help

Hey,

I have a question about the value of some monsters.. I started playing a few weeks ago but I have NO idea to see if a monster is good or bad.

I want to trade and get a good formation but I have nooooo clue what formation is good and wich of my monsters is good or bad..

Ok here is my list of monsters I have... Can someone tell me which one is valueable and which ones are trash??? thanks

List:

My A+ :

Divinity bow clarissa (A+ I got this from 2 clarissa I got from ally invite summon) Psycho down skill
Naga (A+) Slumbering breath skill
Fafnir (A+) Critical up skill

My A :

Scorn black wings (A) decrease enemy attack skill
Sky dragon (A) Increase party attack skill
Beelzebub (A) Venomous breath skill
Armored roc (A) increase party defense skill
Child of Purification (I got this 2 times so I can make A+) Slumbering breath skill
Hell scythe matnis (A) Decrease enemy defense skill
White winged Elite (A) Agility up
Hell Hammer Spider (A) Decrease enemy attack skill
Slaughter Bee Queen (A) Preemptive strike skill
Great horned warrior (A) Preemptive Strike skill

Then I have a few skillless A :

Lust demon (A)
Tormentress Hilda (A)
The first 5 raid bosses from the missions

Sooo can Anyone tell me what they are worth.. and what is the most valueable and what monster are shit..

And also... how can I see for myself if a monster is good or not?

Thanks!!!