A Greatsword (puzzle weapon). After you land a critical hit with that sword, the crit chance increases to 100%. Yup. Guaranteed crits, after the first one. (This is with two of the weapon's puzzles solved. No clue about the third. Hope it does not remove the crit chance, that would be a damn shame!)
I think this is the most powerful weapons I found so far. And you can even get it quite early. Especially once you have the passives that refund opportunity points and reset cooldowns on crit, this weapon is pure awesomeness. There are weapons that add more strength, hence damage, but they can't really beat the utility of this crit chance increase.
Rough guide to get it: Explore Kylearan's tower, where you meet the mage Alguin. One chest has a bunch of recipes that allow you to craft different puzzle weapons using 4 "Elven Weapon Shards" (Yellowish gems you can find. Not too many of them.). I think the chest is in a secret room kind of close to the end, before reaching the elevator.
One of the craftable weapons is the great sword I discussed above. I dont know whether the other crafted weapons get the same effects from solving the puzzle. Or if they are overpowered in different ways.