This is how I'd rank the usefulness of the various SSRs and SRs by element, that are eligible for voting and obtainable from Gatcha (first is highest priority).
SSRs
Fire: Amaterasu, Svarog
Water: Cthulhu, Ryu-Oh
Wind: Gaia, Hastur
Thunder: Raiko
Light: Sol
Dark: Hades/Satan
SRs (bolded=important for their element, even for those with multiple SSRs)
Fire: Brynhildr, Raguel
Water: Triton, Belphegor
Wind: Oberon
Thunder: ...
Light: Diana, Belobog
Dark: Nyarlhotep