The best I ever saw was BOM-EWR when we flew due north passing within 100 miles of the pole. I woke up somewhere over the arctic and it was like the entire sky was filled with green and orange fire. Very disorienting at first, then so incredibly cool that it cost me a few hours of sleep...Remember the aurora varies with the sunspot cycle (11 year) and won't be seen unless it's pitch dark outside (i.e. winter). Also, they center not around the north pole but the magnetic pole over northern Canada (80N, 73W). Any flight path going within 20-30 degrees of this location in the dark would have a reasonable possibility of seeing them if they're occurring, just be sure to book a window seat on that side of the plane.
Another very cool night flight sighting came in 1997 on an FRA-KIX flight. This was during the time of the Hale-Bopp comet (the one associated with the Heaven's Gate mass suicide). As we were flying over Mongolia, I saw the comet due north of the flight on the horizon pointing vertically like a knife into the heart of Ulaanbaatar. Very mystical!