Currently there's chaotic maul, chaotic rapier and chaotic longsword. For my calculations, I'll assume that you're wearing a dragon defender with the 1h weapons, and I'll use the info from the wiki, so there's a chance it'll be incorrect, but oh well.
Dragon defender stats (so I won't have to mention them again and just add them up):
+20 melee accuracy
+10 melee strength
Chaotic maul:
Attacks every 3s, is 2h therefore you can't wear a dragon defender.
The stats:
+139 melee accuracy
+123 melee strength bonus
123 / 3 = 41 strength bonus/1s (the higher the number, the better.)
Chaotic longsword:
Attacks every 3s, is 1h meaning you can take a dragon defender with it.
The stats:
+116 melee accuracy
+102 melee strength
112 / 3 = 37,3 strength bonus/1s
Chaotic rapier:
Attacks every 2.5s, is also 1h.
The stats:
+92 melee accuracy
+82 melee strength
92 / 2,5 = 36,8 melee strength bonus/1s
If comparing the DPS, Chaotic maul wins by quite a lot. Chaotic longsword comes second place, closely followed by chaotic rapier. But keep in mind that the chaotic maul is two handed, therefore you can't have a shield when wearing it. Therefore, if you want a shield, chaotic longsword would be the best option since it offers better accuracy and DPS. The only reason why you should buy a rapier is if you want the bonus attack speed.
Edit: The weapons also give 10% damage and accuracy against your slayer task, so the more strength bonus and accuracy you have, the more effective it is.
Edit #2: Chaotic weapons are also degradeable, so the faster it attacks, the faster it loses it's charges. Chaotic rapier attacks 0.5 seconds faster than the other two options, so it loses charges around 15% faster.