Overall game plan for summons:
Your main summon essentially determines what type of weapon grid you will build. Your main summon has two different ways it can help your stats through its "aura". Either it can give an across the board buff to characters/attacks of a certain element, or it can multiply the buffs you get from the weapon skills in your grid. Most summons from events and gacha are of the first type, and will be something like 40% increase to fire attack or dark characters attack or something like that. These summons are good to have but will pretty much always be filler that you will eventually replace, at least as your main summon (with possible exceptions for the highest Bahamut/Lucifer tier but maybe not I don't have any of them so I haven't paid much attention there).
The second type of main summon, that multiplies your weapon skill, will be your main goal. Omega (magna in JP version and both terms are used interchangeably) summons for the main six island bosses (Tiamat, Leviathan, etc.) are relatively easy to farm and will quickly become better than any event or "showdown" summon like Diablo once you use them along with the corresponding omega SSR weapons on your grid. The weapons and the summon drop from the same place, from fighting the Omega raid for that boss. But you do need both the weapons and the summon for these to work right.
The aura isn't the only thing to pay attention to for summons though. There's also the stats for the summons, including the four you choose for your support grid. The Omega summons don't have great stats, so they are not good long term choices for your support summon grid. Diablo, Ifrit, and Anat all have good stats and can be farmed (casino for Anat). So that's another avenue to pursue.
