Okay, quick take: to my knowledge there isn’t a straight-up item called an 'onyx bracelet' in Old School RuneScape that functions like a combat bracelet with fixed stats. What people usually mean when they mention onyx in OSRS is the gem itself (used for high-level jewellery) or high-end projectiles and gear that carry the onyx name. Because of that confusion, the useful way to think about 'best stats' is to decide what you want the bracelet (or jewellery) to do for your build.
If you’re chasing raw melee damage, prioritize strength bonus and the highest slash/crush/stab attack bonus relevant to your weapon. For ranged, seek maximum ranged attack/strength. For mage, magic attack and negative magic defence don’t help—focus on gear that boosts spell accuracy and power. Defence bonuses are secondary unless you’re doing tanking content. If your goal is PvP, look for a mix of offensive bonus and defence where needed, and for PvM prioritize damage-per-second and special effects.
If you actually meant an onyx-containing piece like a ring or amulet, compare its numeric melee/ranged/magic bonuses on the Wiki to other jewellery in slot — whichever gives the biggest offensive stat for your style is 'best.' If you want, tell me the activity (bossing, slayer, PvP) and I’ll point to exact swaps and cheaper alternatives — I love theorycrafting that stuff.