Up until a few years ago, I would have assumed the 744, but with the A380, 77L, and the 77W making longer flights, what are some of the hour to cycle ratios for these airplanes? If we could forecast to the time of retirement, what do you think these airplanes' numbers will look like?
"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
Well it all depends on airline, but some rough numbers that I have seen are
DC9 1hr per cycle
737 Classic 1.3 hrs per cycle
737NG 2hrs per cycle
747 8 hrs per cycle
757 2.5 hrs per cycle
777 6.5 hrs per cycle
While the 777 has the longest range, there are a lot of airlines operating them on shorter routes, especially within Asia which brings down the average hours. The 747 though is still rarely used on the shorter flights, so it on average has longer routes.
If you have never designed an airplane part before, let the real designers do the work!