Tulsi is the Holiest Tree, said by Lord Shiva.. this sentence is mentioned in the Padma Purana.
Once Kartikeya asked Lord Shiva about the holiest tree, which was capable of giving salvation.
Lord Shiva replied
The Tulsi plant is supreme among all the vegetations. She is very dear to lord Vishnu and fulfills all the desires of a man. Lord Krishna dwells near the place where there is a Tulsi plant. Spirits and ghosts never dare to venture near the Tulsi plant.
If a man attaches a Tulsi leaf to his Shikha at the time of his death, he is liberated from all his sins. One who worships Lord Vishnu by offering Tulsi-leaves attains salvation.