This remains a controversial area but a recent major study (Qiu Y.L.
et al. 2006) concluded that liverworts are the most primitive land
plants. It also concluded that hornworts, rather than mosses, are the
nonvascular plants most closely related to vascular plants.

Qiu Y.L. et al. 2006. The deepest divergences in land plants inferred
from phylogenomic evidence. Proc Natl Acad Sci U S A. 103(42):
15511-15516.
Abstract - http://www.pnas.org/cgi/content/abstract/103/42/15511
Full article - http://bryophytes.plant.siu.edu/PDFiles/PNAS.pdf

David R. Hershey