Very few people will argue that RadOnc isn't competitive, and it IS impressive to have 6/6 match. However, few will argue it is more competitive then Plastics and Derm. As I find little use in generalizations, I'm sure you would agree, I submit the following from 2011 match averages for US Applicants (
http://www.nrmp.org/data/index.html):
Plastics: 249 Step I, 45.9 AOA, 3.8 Research Xp
Derm: 244 Step I, 50.8 AOA, 3.7 Research Xp
RadOnc: 240 Step I, 31.2 AOA, 4.2 Research Xp
Just for comparison, Oto which is also considered a very competitive field:
Otolaryngology: 243 Step I, 41.6 AOA, 3.5 Research Xp.
While RadOnc is undoubtedly competitive (Top 5 in my Opinion), it is not the most competitive as your classmate stated, unless one considers average research experiences the gold standard for comparison. Not a commonly held belief. I only make a point because I believe premed applicants should be supplied with the most accurate information possible in order to make an informed decision. I don't want to hijack the thread with arbitrary discussion, so this will be the last I say on this particular matter.