Personally, I found the best weapon is the broadsword (especially one with special Break Attack - Hurricane Slash).
I used it for attacking multiple enemies at the Kingshead formation or for invasion, for quick leveling as there are more reps gained compared to defeating a single enemy with the same amount of time. My pirate had its sword leveled from 11 to 17 in just this morning due to the massive rep and time saved using broadsword. Yes its a powerful weapon, but it does in no way reduce your reps due to the above reasons.
If only we had special cutlass that allows us to save time on leveling too! I'll say get yourself a broadsword at any blacksmith and start plundering for better ones right away.
Powerful Break Attack using a Military Broadsword for Level 13 Sword.
