What is Doctor Who password?

If you are referring to How the Doctor gets into the Tardis and no one else can, then the answer is two answers. :-)

1.At first it was a key. Only unless you had a key can you get in the Tardis. & 2. Later on in the NuWho, River Song teaches the 10th doctor what another higher regeneration number doctor taught her on how to open the Tardis, with a snap of His fingers. (Older him teaches younger River, Older River teaches it to Younger him -- That's time travel for you..... all Timey Whimey)

In the episode "The Doctor's Wife," Amy and Rory discover the archived control room of Tennant's Doctor. The "passWord" to enter is a telepathic impression: the color crimson, the number eleven, the feeling of delight, and the smell of dust after rain.

The computer passWord for Clara's wi-fi is RYCBAR123.

In "World War Three," the Ninth Doctor told Mickey Smith to use the passWord "buffalo" (originally being "bison" ) to access the UNIT website. To allow remote flight of UNIT missiles the passWord is/was "bad wolf". The UNIT dot ORG website secure login was buffalo - the secure section has been removed.