It really depends on the monster or spawn your fighting. Look up the spawn or boss (Uoguide or Stratics) you want to fight and customize your weapon accordingly. There is no one fits all.
Generally for Undead Slayer and Arachnid Slayer you'll be wanting 100% Fire Damage.
Rikktor and Barracoon Spawns you'll want 100% Cold damage with Dragon/Reptile or Repond Slayer.
Against Oaks and the Wood dwelling creatures you'll want 100% Fire Damage or Fey.
75% of the monsters in the game are vulnerable to either Fire or Cold. Only a small percentage are susceptible to Energy and Poison Damage. However there are some very strong adversaries that are:
Putrifier you'll want 100% Energy Damage Demon Slayer. (You can use this weapon on Balrons as well)
Rend you'll want a 100% Poison Damage Dragon Slayer.
It's well worth the investment building up an arsenal of elemental weapons even if you have a weapon with AI on it.