I used to fly weekly for work so I've had a few rough ones.
One of my worst experiences wasn't really an airline issue, but more so the weather. I had an early morning flight from O'Hare to LaGuardia and the weather was awful all day. It was late January / early February and we were getting some nasty wind and thunderstorms. My flight got cancelled so I rebooked to a later one, that one got cancelled, rebooked, cancelled, rebooked, etc. I had my flight number changed 4 or 5 times. Finally around 8 PMish, the last flight headed to LaGuardia got the go ahead and luckily I was able to get a seat. The plane took off and the turbulence was rough for the whole flight, the flight attendants stayed strapped in and didn't attempt to do drink service. I checked a Pelican case with me stocked full of tools and of course it never made it on the right plane. I ended up getting my tools delivers to my hotel the next day. Anyhow, I landed in New York around 11 pm, got outside in the pouring rain and stood by the Avis shuttle bus stop so I can get a rental car. After awhile of waiting, no busses came, so I started walking towards the rental agencies. Walked through the pouring rain and actually ended up getting picked up by the Enterprise bus as I was about halfway. He made an exception for me and dropped me off at Avis where I had my reservation. Got there just as they were getting ready to shut the doors for the night, but luckily got a car. All in all just a miserable day and night.