Strongest Item you were allowed to play with . Hail DM

[King of Nowhere]

not exactly an item, more like a plot-related blessing, but my character was given the ability of shapechanging into an earth elemental of equal hit dice.
and my character is a monk. it deals 12d8 damage.
however, it only lasts one minute per level, and my gear gets melded into the form unless i take it off and put it back on afterwards. so i can only use that shape if i am prepared for a big fight. or i can use it in a pinch to escape with earth glide.

as a dm, i gave the players a weapon with the following stats:
mutating: can take the shape of any weapon. damage 1d8 if one handed, 2d6 two handed
crit 18/20 x3
+10 enhancement bonus
cut the weave: it can dispel one spell with a touch attack. you can mix dispelling attacks and normal attacks as you choose in a full attack
evil aura: all enemies in an 18 m radius are shaken unless they pass a DC 30 will save. if they fail by 10 or more, they are panicked instead. furthermore, all creatures (allies included) are sickened, and must pass a DC 30 fort save to avoid being nauseated. you can focus this second effect on a single creature (thus sparing your allies), in this case it needs to save against DC 35 to avoid being nauseated.

although it was supposed to be the ultimate artifact weapon, bestowed on the party - with a bunch of other artifacts - as a last resort to stop an unstoppable villain, and the party had to earn it. it was also a high power campaign where the enemies also hit very hard. this weapon was strong, but it didn't break balance.
no, it was the bunch of other artifacts i gave - giving all the party a +8 enhancement to armor, shield, deflection, natural armor, saving throws, wisdom, constitution, dexterity, plus other things - and the +4 to all stats due to absorbing the power of a dead god, that threw the numbers completely off the rails.