Okay so here's the thing, I'm in Post-game, I have beaten Gabriel Celesta, and i'm now in the wandering dungeon, my team is Edge, Reimi, Arumat and Sarah, I have strong weapons (Quadplex Scythe, Moonstone Sword, Evil Spirit Bow and Unicorn Lance) and good armors for anyone but Reimi (Valiant Mail, Seven Star Cloak and Holy chainmail) But I have only crappy accessories beside the two collosseum medals (one on Arumat and the other on Sarah) and the Ashlay Jewel, So what are the best available to me? I know I can go buy the Tri-Emblem, but I didn't once found Santa in the Wandering dungeon and Tri-emblems only fill the wrist or neck slot so that leaves an other spot free...
Also could you tell me what's the best thing to synth in weapons/armor/accessories? I know that in Star Ocean 3 one had only to synth 8 Boots of Prowess in everyone's armor to have a huge up in defense, but I doubt there is something like this in this one no?
Now is the time to choose! Die and be free of pain or live and fight your sorrow! Now is the time to shape your stories. Your fate is in your hands! Auron, FFX
Basic tri-Emblems ain't that great, but you wanna buy at least eight of them (if you stumble upon Santa) to create a so-called Super tri-Emblum. Take the emblum and synth all the emblems on it that you can to have the highest stat boosting accessory possible in the game. Dupe it a couple times (just a single Magical Clay each on rank 10) for all characters and you got that one down.
Second choice for ideal postgame accessories would be something to resist multiple ailments. You definitely want anti paralysis (Gabe and EQ). Anti stun and silence would be the next best picks. the last factor is really up to you.
Ideal are the wepons and arnor with the highest base stats AND four empty factor slots to use during IC, so keep that in mind when picking the fitting gear for each character. Laser Weapons/Suits, while not having the highest stats, are still universally useful since everyone can wear them and they have the four free slots to customize. For some characters that makes them more useful overall than their supposed "ultimate" gear.
Balance the factors used for your equipment to max out ATK and DEF (or at least come close to it). The sticky thread has more details where to get the required items. The rest depends on your playstyle and the character in question. Some of the most useful stuff is "Add hit chance per attack", "Increase critcal hit rate", "Stuns enemies", "Nullify HP damage 25% of the time", "Rush gauge +2 (or 3)", "Max HP+20%", among some others.