From what I know, and reinforced by what some USAF folks have said in this thread, in the Air Force the USAFA gets the bulk of initial pilot slots for the service while the 'leftovers' go to ROTC. USAF OTS is their 'spigot' too but they often get few to no pilot slots at times from what USAF types have told me.
For the Navy they seem to spread out the pilot slots between the USNA and ROTC equally, with OCS again being the 'spigot' but it usually has a decent number of pilot slots that folks off the street can actually get.
So to answer your question, USAFA grads have probably the best shot at getting a pilot slot by the numbers but both USNA and Navy ROTC grads have generally gotten a pilot slot over the last few years, if they are qualified. That is a function of how the Navy spreads its billets more equally among the USNA and ROTC programs in addition to having a more diverse set of 'warfare' officer billets to include submarines and surface ships than the USAF.