My worst one would be Nottingham to Skegness in the summer quite a few years ago. Me and my mum went and we also took her friend with her children who had never used a train before, or since. We got to Nottingham and there was a single 156 on the platform with a large crowd waiting. Good start I though, having had prior experiance of overcrowding (including 2 summer saturday evening crush loaded 153s back, first time being a big contrast to the quiet 3 car 117 on the way out). Things looked up though when another 156 arrived and coupled on the back, but we were still not able to board. It then became clear why when not long before departure a 158 pulled in, having come from Crewe, and also coupled on, giving 6 coaches, much more like it. Having boarded, and with the train fairly busy but not too bad we pulled out. This is where it went downhill fast.
The departure from Nottingham was very sluggish, and then we made an unschedualed stop at Netherfield, the first station out of Nottingham. The driver left the cab and walked down the train. On returning to the cab some minutes later it was then announced that the brakes on the 158 were not releasing, hence the performance. We were by now quite late and people were getting restless, but it was announced that the fitters had been called out, and the guard had opened one door so people could leave the train for fresh air. The doors in question were the back ones on the rear 156, and from there the smell of brakes on the 158 was strong. The connecting doors had been closed and an attempt already made to sort the problem by uncoupling and re-coupling, but this had not worked. The crew told us that the best solution would be to leave the 158 and continue with just the 156s, but control was insisting on sending the fitters out first. By the time the fitters arrived it was over an hour since we had left Nottingham. They tried various things, including uncoupling and recoupling, with no luck, and eventually as expected we left with just the 156s, leaving the 158 behind. With the passengers from the 158 now moved into the 156s the train was much busier, and 2 hours late. We eventually pulled into Skegness with a few minutes to spair from a 2 hour delay, 2 hours of our trip lost. We also noted that the train an hour behind had been cancelled, and a large load were waiting, this being the first train to leave Skegness for 2 hours.
We made the most of the day, then returned to the station for what turned it into by far my worst return trip ever. On arrival, in plenty of time due to the expected heavy usage of the summer evening departures from Skegness, we found large crowds already waiting and a 150 in the platform. Since it was still some time before departure, I assumed(and hoped) this would either not be our train, or another unit would be coupled to it before departure. This turned out not to be the case, and the familiar scenarior occured where passengers for stations to Boston were asked to go and wait for buses which would be provided, and then the rest of us squashed onto the 150, practically crush loaded (many were standing, and the aisles and vestibules were filled with luggage) with 2+3 seating for the long run back to Nottingham. It was so bad it actually ended up being mentioned on the local news due to passengers complaining, with the explanation given that it was due to an earlier failed train disrupting the service.
For some reason I began to hate 158s as after this, although my overall milage on 158s was nothing special compared to various other types (particularly Turbostars and 156s), my delay minutes due to them are vastly higher than any other type, including 2 more 2 hour delays due to 158s and another due to a gas leak but involving getting stuck on a 158. This is even more so if knock on delays are included. Add to that a number of uncomfortable runs due to air con failures and it got worse. Then again, all involved Central Trains 158s, with one exception (Arriva Transpennine) I've never had an issue with anyone elses 158s. Even so I found them worse than 170s and 156s that I had similar if not higher milage with (not once had a delay down to a 156).