Mainly crap, except sword and shield(arguable), but I just want 1 of each.
Stats level up as character level increases, stats are as follows:
Onion Sword - ATK: LVL*2, ACC: LVL+50, Strength: LVL/3 best weapon for Cecil higher up, arguably for Kain as well
Onion Shield - DEF: LVL/10, Stamina: LVL/3, Prevents: Everything but Berserk
Onion Helm - DEF: LVL/10, Magic DEF: LVL/10, Speed: LVL/3
Onion Armor - DEF: LVL, Magic DEF: LVL, Intellect: LVL/3
Onion Gloves - EVA: LVL/5, Magic EVA: LVL/5, Spirit: LVL/3
So yeah, sword and shield are best two imo, though others can be useful, since max stats are affected by equipment, but not really worth it to lose DEF or EVA or w/e compared to some of the other stuff you can get, ie adamant armors > onion armors, crystal rings/protect rings > onion gloves, ribbons > onion helms, though I'm waiting til my 3rd playthrough to get those, since only onion and adamant carry over.