Verlaat- Great SS (best out of the 4)
Zalzugal- Great SS and also usable with only 1 copy.
Allind- Good SS (least useful out of all them) and okay as 1 copy but you want 2.
Oseon- No one cares this has a SS and usable with 1 copy. Also you already have Marduk.
That being said I say Oseon but up to you and to your needs.
"While Retear in his previous state was certainly powerful, we felt that his skill set was not particularly satisfying to use..." A.K.A. he was too Refess for Refess.