You all are impressive...but, can I add to the OP's question with a little twist? What's the longest flight you've ever taken WITH KIDS?

To answer the OP's question...

I think either Jeju-GMP xfer to ICN-NRT-LAX-DFW

OR DPS-HKG-LAX [and thanks to AA pilot's sick out] -DEN-DFW

OR DFW-ORD-BRU-MOW, return St. Petersburg-ORY-DFW

WITH KIDS...longest trips have been:

(1 yr old and 5 yr old) DFW-ORD-PVG (1st international trip and my 5 yr old asked the FA to make her some chicken nuggets for dinner!)

(same 1 yr old and 5 year old, 1 month later) DFW-LGW xfer to LHR-Malaga
and return Malaga-MAD-CDG (stopover to visit friends in Paris) - DFW

Upcoming trip (leaving Friday, yeah) with now 2 yr old and 6 yr old,

DFW-LGA xfer to JFK (6 1/2 hours in NY!), JFK-MXP, drive 3 1/2 hours to NCE

Still not very competitive with some of you crazy folks, but then again, having the kids along adds some excitement to the trip! (And I can add that last, upcoming trip to the Longest Trip with the mother-in-law category, too )
-L

P.S. Adding - My husband and I had to split up on a return last summer from MSP to DFW. My 6 yr old and I spent the night in MSP and flew to DFW the next morning; but my husband and 2 yr old ended up flying MSP-ORD-waited several hours and eventually got onto a much delayed AUS flight; arrived about 2 AM, slept at airport 3 hours and took 6 AM flight from AUS to DFW - and he went to work that day! I think he wins for most difficult trip.