It took me a few minutes to come up with one, since most of my road trips are at least generally pleasant.
However, my trip to a weeklong "Guitar School" for music ministers (I've played in church for 28 years) in the summer of 1995 might qualify as my least favorite road trip to date. This took place in Belleville, IL (across the river from St. Louis).
The school itself was great (learning new techniques, bonding with other musicians from around the country, etc.) and something that left its mark on me. I also got to see my uncle's family at the beginning and end of the week.
However, I spent the week plagued by what I thought at the time was just a bad cold (but was, in retrospect, a severe sinus infection). And the scenery (or lack thereof) on I-55 and I-70, especially in Illinois...need I say more?
On top of that, I had planned on spending the night in South Bend on my way home...except that the Brickyard 400 at Indy, which I had never heard of, was that day, so no hotel rooms in South Bend or Elkhart were available. Long story short, I drove all the way through to Plymouth, MI, arriving at 3 AM.
The final coup de grace...After having visited a few places on that trip that someone I had been dating had recommended (she had lived in metro St. Louis for a few years), and presenting her with a few souvenirs, she unceremoniously "Dear Johned" me less than 48 hours after I got back.