Which are the most important carcinogens in tobacco smoke?
This is not an easy question - as tobacco smoke contains something like 5000 different chemical components, from which 70-100 are carcinogen. The list includes: Oolycyclic aromatic hydrocarbons (PAHs), N-nitrosamines, aromatic amines, aldehydes, volatile organic hydrocarbons and metals. I haven't found articles which specifically concentrate on the carcinogens in saliva, but my guess would be that you find them all because of the "application route" of the cigarettes.

Most of these carcinogens first need to be processed from the livers cytochrome P450 system to reach their carcinogenic potential. The enzyme complex converts these substances into forms which can then covalently bind to DNA. 

A list of several different of the carcinogens can also be found in the Wikipedia. For further information see the references.

References:


Hazardous Compounds in Tobacco Smoke
How Tobacco Smoke Causes Disease: The Biology and Behavioral Basis
for Smoking-Attributable Disease: A Report of the Surgeon General.
Tobacco Smoke Carcinogens and Lung Cancer