Assuming I can pick from any SSR weapon for each element? In general, I'd be looking at triple skill weapons, so kamihime release weapons first... failing that, then I'd look at the draconic eye exchange weapons.
Fire: Da Ji's weapon becomes Strength (M)/Rush (S); Strength is Assault/Defender
Mars' weapon becomes Assault (L)/Rampart (S); Rampart is Defender/Vigorous
Fire Beelzebub's weapon becomes Defender (L)/Twin Edge (S); Twin Edge is Assault/Rush
Ra's weapon becomes Strength (M)/Stinger (S)
Water: Sarasvati's weapon becomes Strength (M)/Stinger (S)
Neptune's weapon becomes Twin Guard (M)/Assault (S); Twin Guard is Defender/Rush
Wind: Seth's weapon becomes Defender (L)/Avalance (S); Avalance is Rush/Barrage
SSR Cybele's weapon becomes Assault (L)/Tri-Guard (S); Tri-Guard is Defender/Barrage
Thunder: Marduk's weapon becomes Strength (M)/Exceed (S)
Athena's weapon becomes Defender (L)/Strength (S) (...yes, that's Defender (L) and Defender (S) in the same weapon)
Mammon's weapon becomes Assault (L)/Rampart (S)
Light: Eros' weapon becomes Strength (M)/Elaborate (S)
Shamash's weapon becomes Strength (M)/Pride (S)
Aten's weapon becomes Defender (L)/Resilience (S); Resilience is Assault/Ascension
Dark: Nephthys' weapon becomes Strength (M)/Stinger (S)
Pluto's weapon becomes Defender (L)/Avalance (S)