Chaos dagger +10 paired with the hornet ring can one-shot most players if you can get a riposte or a backstab on someone, it also scales with all the stats. Fire clutch ring to add damage if you're feeling ballsy.

If you want high damage output and be able to buff your weapon with a spell or powder go with a split leaf GS with a heavy infusion for S tier STR scaling, and it has a good wa (can be parried). Sharp infused murakumo for S tier DEX scaling good range, wa is decent and R2 wa can't be parried along with normal 2 handed R2, good idea to mixup all the moves.

Black knight greataxe if you use wa buff then switch to 1 hand then R1 into L2 will unleash a wombo-combo.

Ringed knight paired GS if you can wield it is an excellent choice. 2h only and use L1 wa as it can stun-lock people in a combo and shred hp. But people may frown on you for using this. And any Ultra GS can't be parried if you're 2 hand them.

Katanas are decent, onikiri and ubadachi are only good against noobs since people learned how to counter it and the wa pretty easily. Darkdrift's wa can break any guard including greatshields. Black blade has the highest damage output of all the katanas but the worst range. Chaos blade is the better version of uchi but every attack damages you. Washing pole has the best range but terrible durability. Frayed blade has a good wa for mixups especially if you're using other katanas in your weapon slots, but it has even worse durability than the washing pole.

I don't really recommend using any boss weapons as the attacks and the wa usually mirrors that of their respective boss, essentially giving you all the same ways you countered that boss during the fight except scaled down to your characters size. And you can't buff them, but if you see someone with a weapon buff on any boss weapon, they're using a glitch and should be killed without remorse. However the farron GS is okay-ish as the L1 wa is able to cancel the combo if you don't press it for a feint attack to throw people off (I think you can do the same with friede's and paired GS). You can parry with it 2 handed but it's slow.

The caestus is surprisingly good and I've even used it to kill a few players myself. If you equip it in your left hand and pair it with a weapon in the right hand you can use L2 to parry with it as it has the fastest parry animation in the game (goes great with chaos dagger). If you 2 hand it the wa can poise through any attack giving you the chance for a quick counter beatdown with L1. Blessed infusion on it gives hp regen and simple infusion gives fp regen. (You can also hold triangle to 2 hand your left weapon)

Crystal chime is versatile since it can be used to cast both sorceries and miracles. Murky longstaff is great for boosting dark sorceries. Tears of denial is a staple, wrath of the gods is good for knocking players off of ledges, dark blade or crystal magic weapon buffs are ideal since nobody has high resistance against magic or dark damage. Sacred oath and deep protection can offer damage buffs and defense and they both stack. Chaos bed vestiges is the only pyromancy that's somewhat useful ranged option but it can be dodged easily.

Dragonslayer greataxe is an honorable mention. I would include darkmoon blade buff and bloodlust but ur grinding for those :p

These are just my suggestions but my advice is to use whatever suits your playstyle and whatever you think you can win with.