Roadster - the reason the RFC and the RNAS are counted towards squadron service is because of the precedent of antecedant units being counted. The squadrons didn't all form on 1 April 1918 - they existed and simply changed their arm of service to RAF rather than RFC.
Strictly speaking, of course, you're correct - as there was no RAF until 1 April 1918, then all the squadrons extant at 0000 that day are the oldest RAF squadrons, making the original question meaningless.
The answer to the original poster is then along the lines of 'every one on the current ORBAT bar 617' which doesn't win the wager for either party...