The best chances, IMO, to get a AF pilot slot are in order: USAFA, AFROTC, "pledging" a ANG/AFRES unit, and OCS. Only the last two allow you to "know" that you have a UPT slot before you sign on the line. However, 80% of annual pilot slots go to the first two. You pays your money and you takes your chances.
I have no direct knowledge of Navy/Marines/Coast Guard/Army slots. I do know however, that the mix of rotary wing to fixed wing initial training slots are much higher for all these (Navy/Marines probably 60%RW/40%FW, Army is 98%RW, CG is probably 85%RW) than that of the AF (90%FW/10%RW). Might make a difference on how you chose a service, depending on whether you want to fly Helos or airplanes.
But, in all cases, I reiterate the adage that I and MikeD have been saying for a long time. Go into the military to fly because 1) you want to be a military officer, 2) you want to fly military aircraft in a military mission. Wanting to go through the ordeal of UPT and the subsequent years of military commitment because you are looking for a cheap way to get flight training for a civilian career is sure to be a disapointment both personally and professionally.