Well there are many powerful objects in the Whoniverse;
Many are Gallifreyan artefacts that carry the name of Rassilon. These included the Sash of Rassilon, the Rod of Rassilon (TV: The Deadly Assassin), the Coronet of Rassilon (TV: The Five Doctors), the Harp of Rassilon (TV: The Five Doctors), the Crown of Rassilon (TV: The Invasion of Time) and the Seal of Rassilon, a symbol used as a mark of Time Lord authority which appeared as a motif in many Time Lord designs (first seen in TV: The Deadly Assassin, named in TV: The Five Doctors)
These were stored in the Capitol and, except for the supremely powerful Great Key,were made available to the Lord President of the Time Lords. (TV: The Invasion of Time). The Black Scrolls of Rassilon, contained forbidden, arcane secrets (TV: The Five Doctors).
The Ring of Rassilon capable of conferring immortality upon the wearer, resided in the Tomb of Rassilon in the Dark Tower located in the Death Zone. (TV: The Five Doctors)
BUT what is said to be the most powerful object in the Whoninverse is the Moment.
The most powerful and most dangerous weapon in all of creation (not the T.A.R.D.I.S.), described as the Galaxy Eater it is capable of whole galaxies within in a single moment, hence its given name.Able to create tears in the fabric of creation called time fissures that would allow people and objects to pass from one time period to another, possessing a trans-dimensional awareness of the past and future. It also had a sentient interface with telepathic abilities, enabling it to read the thoughts and memories of those who intended to use it.
The interface also had the capacity to take the forms of others as a holographic projection. As it was a Time Lord construct, it was not limited to a linear grasp of time and space- it could select the forms of people from an individual's future they had yet to meet.It can also change appearance from a box full of gears, to a box looking station with a big blood red ruby button on a stalk, with a base similar to the shape of a rose petal.
The Moment was the final creation of the Ancients of Gallifrey. It had an operating system so sophisticated and advanced it had become sentient and developed a conscience. This is why it was never used, as the Time Lords feared its moral judgement). When attempting to initialise it, the War Doctor expressed unfamiliarity with the Moment's controls and operating system. (TV: The Day of the Doctor) According to another account, however, the Eighth Doctor created the Moment from a modified Key of Rassilon and De-mat Gun during the Time War. (COMIC: The Forgotten, Don't Step on the Grass)
Until the Doctor stole it, the Moment had been locked away along with other forbidden weapons,collectively known as the Omega Arsenal, in the Time Vaults. While all the other weapons were used against the Daleks during the course of the Time War, the Time Lords refused to use the Moment for fear that it would punish anyone who did so.
(RE:multiple TDC/OTHER entries)