Worst delays I can recall have both been at LGW.
First was a transatlantic to MCO, we were due to travel on a Laker DC10-30 (Laker Mk II, American registered aircraft), but the 10-30 was tech, so they sent a 10-10 instead. As a result, we were warned that we would have to tech stop at one of the Canadian stops on the extreme eastern end, can't remember which it was, other than it was darn cold, minus something, (October) and we were all dressed for Florida, and had to walk across the ramp to the terminal building.
Anyway we all checked in, and at the appointed time, we boarded. Then we sat, and we sat, and we sat, watching all sorts of frantic activity outside on the ramp, as baggage cans were moved in all directions, unloaded and reloaded. Eventually, we were told that we were going to be departing shortly. Well, nearly. At this stage, we'd been sat in a full DC10-10 for close on 3 hours, and the humour was not improved any when we were told the reason for the delay. which was that (Pre 9-11 but still an American flight) the pre check in security check, and the check in bag count didn't agree, so the handling agents had been required to strip every can and ID the bags. Turned out that some *&&^^%%% at the check in had double tagged a bag. So, some 3 hours and a bit late, we pushed.
Start 3. Taxi to the hold. Wait a bit longer. Captain comes on the PA. Sorry folks, we've got a slight problem here, one of the gauges on the fuel system is not working correctly, we're going to have to go back to the gate to get it checked. Back we go, another 30 minutes, and we eventually launch. Cross the Atlantic, fuel stop, land in MCO about 5 hours late, which then caused "fun" as an entire DC10-10 worth of people are all boarding coaches to go to the same car hire company.
That's another hour and a bit in the queue, though we did quite well out of it, there was only the 2 of us that trip, we'd booked a group A, that was all we needed, the car hire rep did just about everthing that was legal to try and get us to upgrade, which I declined, and in the end, we got a 4x4 Jeep Cherokee, that was all they had left at that stage
In hindsight, the "fuel guage" was probably a good bit more complex, for the first sector to Canada, the IFE in the rear of the cabin was U/S, and the cabin crew were struggling to get meals hot and the hot drinks were'nt very. After the fuel stop, it was strange how everything was working 100%, I suspect that something more fundamental had tripped, but after looking at it, they'd decided to launch anyway to avoid all sorts of hassles with hours and the rest of the related aggravations.
To be fair to Laker, the trip back was a lot better, the 10-30 we were supposed to have had arrived, and it was configured in 3 classes, even though it was a charter, so some lucky passengers ended up in effectively first class. Yeah, we were lucky, and it was a very well configured 10-30, each leather reclining seat had it's own TFT screen IFE, and this was a good few years ago now, so we had a very pleasant flight back to LGW. It helped, as my wife had picked up a nasty bout of food poisoning, so a comfortable spacious seat made her flight a lot more relaxing.
Worst delay ever for me was also at LGW, I came in from the States on an overnight, and arrived in to LGW about 0630 ish, and was due to go on to DUB on a Cityflyer 146 at about 10:00. In the absence of anything else to do, I checked in, and then went through to departure, along with quite a few other people from the same flight.
There we sat, and sat, and sat, with no information or anything else from Cityflyer or the handling agents. By 14:00, I for one was starting to become distinctly hostile, not because of the delay, but because of the appalling bad attitude we were getting from the neanderthal at the Gatwick Handling desk, who was telling us NOTHING. Initially, we were all polite, and reasoned, but we got nothing in the way of information. After a while, the temperature rose, and along with it voices, but we might as well have been talking to a brick wall, the response was still identical.
The reason we were getting so irate was that we knew exactly what the problem was, DUB had been temporarily closed because of industrial action by the fire crews, which caused all sorts of knock on delays. They were telling us nothing, and provided nothing, not even refreshment vouchers, and at this stage, we'd been in airside departure for close on 6 hours.
By now, there was a group of very unhappy passengers around Gatwick Handling desk, and the desk clerk still refused to do anything, and would not even call a representative of the airline. The individual concerned came very close to being flattened by a couple of people, and even then he could not see that it was his total lack of respect for us that was causing the problem.
In the end, we got out very late in the afternoon. I decided then I was never flying Cityflyer again. I haven't.