Damage calculator (Eidolon buffs comparison tab)
You can change the numbers with a grey background to suit your team configuration by downloading your own copy of the spreadsheet.
The eidolons buffs being compared are:
+40% elemental DMG, dual element (first 3 event eidolons)
+40% character ATK/+20% HP, single element (e.g., Jormugand)
+90% character ATK if team contains characters of three elements, single element (e.g., Lilium)
+45% elemental DMG, single element (e.g., Reiki)
Since you can bring two eidolons, I also included the combination of a character ATK and elemental DMG buff, which has a degree of synergy.
Note that the poll doesn't include the last two options because they're not currently available in the Nutaku version and will not be available for a long time.