Strongest Item you were allowed to play with . Hail DM

[vasilidor]

In an early d20 game I made a fighter that used the dual blade. The GM decided to give my character a great sword that was meant to kill gods.
I actually felt disappointed, because I wanted to use the cool dual blade.

[Awakeninfinity]

One of my DMs gave me a +5 Holy Sun blade that allowed me to smite evil once/day

I was a support gish with a celestial theme...

I consider it stronger than the book of infinite spells I got that same campaign.

Which is a testament to how important it is for a DM to know their players.

[Vaern]

We once convinced our DM to let us put a permanent Gate spell cast inside of a Bag of Holding, with the other end of the gate anchored on our ship. The main idea was that the bag's capacity would be irrelevant, as all of our loot would simply pass straight through the bag and end up stockpiled on the ship back on the Material Plane anyway. After a random off-hand joke, though, we eventually decided that any time a player was late or absent their character would conveniently disappear into the bag and find themselves "helping out the crew back on the boat," regardless of where the party was or what they were doing. The character could then simply crawl out of the bag to rejoin the group at literally any time whenever the player finally showed up.
The end result was that we were able to add and remove player characters from a scene at-will without significantly impacting the continuity of the story, making it possible to continue a campaign when a player was absent from the table without worrying about what their character should be doing in an encounter. The Gate-Bag shenanigan literally let us play the game more often, which as far as I can tell is stronger than any published magic item or artifact.