Which method of seed dispersal is the most common among the seed plants?
This paper from Herrera (1989) investigates the role of endozoochory in Angiosperm diversification, and it has a table that can help answering your question, since it deals with all seed plant families:

 

As you can see, among Gymnosperms there is no question: endozoochory is the most common mechanism of dispersal.

Unfortunately, things are a bit more complicated among Angiosperms. Exclusive endozoochory is not the most common mechanism (27.1% versus 72.9%). However, if we take into account that the plants that perform endozoochory are actually the sum of the first and second ("mixed") columns, the total is 50.9, which is more than 50%. Of course, there is a mathematical possibility that another method, together with the "mixed" column, also surpass 50%. 

Finally, it's worth mentioning that the authors are calculating the frequencies of endozoochory only. If one adds epizoochory to that number (zoochory = epizoochory + endozoochory), the frequency of zoochory is even higher.



Source: Herrera, C. (1989). Seed Dispersal by Animals: A Role in Angiosperm Diversification?. The American Naturalist, 133(3), pp.309-322.