There are some SERIOUSLY good photographers here, dozens of which could have my vote.
Of them all, here is my current favorite: View Large View Medium
Photo © Markus Herzig
BUT, there are three other awesome shots worthy of at least a mention, most notably Wietse's "Criss Cross 340 Contrails" shot referenced above (one of the best photos I've ever seen).
Also, Bronko's close-up of the landing gear of a 777 as it touched down, and the "good Morrell's (David - Avid Creations)" up close and personal shot of a Singapore Cargo 747 on the ramp.
There are dozens and dozens of other shots that I have been proud to have as my desktop wallpaper over the years (thanks Sam, Joe Pries, and Mr. De Bevere).