The best mileage run I did was from London to Muscat in First Class on British Airways. This was part of my RTW oneworld First Class trip to try all oneworld First Class product.

What was interesting was the fact that when I arrived in Muscat, I went to transfer desk to get my return boarding pass to London and had requested the agent to make sure that my lugguage that had arrived from London will be transfered onto the aircraft. You should have seen his face when I said that. He was absolutely horrified to learn that I flew First Class down to Muscat and then back to London on one single trip. I guess I wasn't going to reveal my secret mileage run trip but I had no choice as I had a check-in luggage which I wouldn't want to store it in LHR (expensive).

I would not recommend anyone taking this trip in Economy Class on BA as it can be a rather looooooooooooooooooooong flight to earn those miles in cattle class.

Have a pleasant mileage run.