Gotta have the circular slide rule or I'm not taking it flying! Can't see how anything can be called a "pilot's watch" without one.
Additional timezone kept at UTC (GMT, more or less) regardless of what you do with the primary set time is also a must have. Additional time zones easily available is a nice bonus.
While I love mechanical watches, I'll use quartz for everyday use. I love exact timekeeping without constant setting more.
Currently, the Citizen Skyhawk series meet my requirements the best. However, the quality isn't impressive and the customer service (at least my side of the pond) is nothing short of horrible... so great while they work, but don't expect anything on the warranty.
Love to see beautiful watches, and get recommendations on other watches for actual use.