Damn... that's one hell of choice.
Everything is interesting and (IMO) still usable.
I think for this choices, it depends on what file you want to play.
Verlaat: If you have dream playing Refess Lv2 Royalty Rush or Eternal Meh--er... Morning.
Zalzagul: Option for Mid(?) and Big Lawtia I guess.
Ain: If you have dream playing Beltorat Kingdom.
Galdirea: If you have dream playing wacky Galdirea file.
If you already have several playable Beltorat and random Dragon, I think Ain is the best and it's pretty strong too.
And I'm not mistaken, for Ain 2 copy is enough.
I personally prefer Verlaat though, cause Refess.
"Humans fall to corruption.
So too do saints and heroes.
There is no stopping it.
That in itself is salvation."