Mine is definitely landing, especially on a UA flight and listening to the ATC system. I live in Chicago, and am constantly amazed at how the ATC'ers can bring everyone in safely. And at the same time I enjoy listening to the flight crew, and watching and listening as the flaps extend, the wheels drop, and the plane comes back to earth.
But equally memorable are particular flights. I used to live in DC, and would love to take the last USAir or Delta Shuttle to LGA, especially on a clear night with an approach from the South, with Manhattan to the left. Or early morning flights into National from O'Hare on AA or UA, flying low over the hazy Shenandoah Mountains, and, if lucky, landing on Runway 18. I also like short jaunts on the USAirways Express Dash-8s. It's a great plane, feels solid, and flying around the East Coast I saw a lot of interesting stuff. Like one of the pilot's responses, I also like skimming along the cirrus. My favorite flight, though, was from CLE to DCA about 5 years ago. Late night, first row of first class on a Continental MD-80 or -88. We followed a line of T-storms that must have tracked from NW to SE. The clouds shimmered with near-constant flashes of blue color from the lightning. But we flew a perfectly smooth flight in clear weather. Since I was at the front of the MD, the engines were nearly silient, as was the lightning, of course, and I turned off the reading lights and watched a great show. A really sublime experience, especially for someone who usually sits in coach!