The books, Tolkien's letters, and a section of the appendices at the end of RoTK all explicitly say it's tobacco.

Speaking of Hobbits, Tolkien wrote:

they imbibed or inhaled, through pipes of clay or wood, the smoke of the burning leaves of a herb, which they called pipe-weed or leaf, a variety probably of Nicotiana [tobacco plant].

Tolkien was a big smoker. You rarely see pics of him without a pipe in his mouth.

Plus, given the Hobbit was released in 1937 in England, it's likely that Tolkien didn't even know what Cannabis was. He'd certainly be unaware of its future popularity/cultural relevance.