Interesting question ! (lol^^)
PSI - Lashiec of course ! It's not the last boss so no problem about it ! And as the monsters didn't have a Crystal to block his magic...bingo !!!
PSII - Neifirst because she is powerful and then Nei will be alive ! Like someone said before : would have been so great to have Nei alive and a team with both of them.
PSIII - Siren because I like too much his design and he seems to be like Wren so (as a Wren fan)
PSIV - Lashiec like in PS I as it's my favourite character in the villain's category in the whole Phantasy Star timeline !! (as my new avatar

)
In fact the same choices as LUCAS made because that's the four best choices for me !