Hi, it has to be the Sub-Surface railway (circle, district, hammersmith & city, met) - everyday there are failures of some type of the main circle loop. Also when closed at the weekend all of them have to close because they share track. Out of these I would say the circle - simply because it is no longer a circle, there is more room for delay.
The Met line and Circle + H&C lines always seems to be near the bottom (if not actually the bottom) of the list whenever I look.
According to this article a 1/4 of LUL refunds last year were for the Met.
LUL stations do display a small poster (usually by the ticket office) which listed % timetabled km operated for each line during a particular time frame. I found this pdf from the TfL site which ranks lines for various performance measures. In Period 10 (11/12/11 - 07/01/12) the Circle + H&C line was the worst followed by the Met at 95.9% and 96.8% of scheduled km operated.
From my experience as a frequent passenger... not particularly so. There are occasionally problems. Most major issues now appear to be due to signalling faults, but there have been some nasty train failures in the Kings Cross/Euston areas, which could sometimes be extreme cases of sensitive edges failing, I suppose.
Autumn last year seemed to be bad enough for sensitive edge systems. It seemed like every departure had to have an added special announcement along the lines of "Please keep everything clear of the doors or we will not be able to move away from the platform - thank you"!
The Circle line is more reliable now than in the past
I would say least reliable is the Jubilee as well as it being the least resilient to running with any problems that another line could cope with.
It's also the only one now to have a Reliability Director!
Indeed, the Jubilee has George McInulty as a reliability director......is this another case of jobs being created for the sake of being created??? Maybe a different thread.
As someone who works on the Jubilee, it is a lot more reliable nowadays. Sure, we still have failures here and there, but they are getting less and less. I am convinced however that during the Olympics, there will be a high profile train breakdown/tunnel evac at some point. Maybe I am being negative. Like I said the Jubilee breaks here and there, but boy are they proper failures!! Such a busy line and a bit of delay and troubl soon flares up quickly.
As for the other lines, yep the Circle I would say, so much at the mercy of other lines whose track it shares. The Vic can be awkward, usually infrastructure failures around Brixton.
The Northern has improved immeasurably in recent years. Signal failures and faulty trains are much rarer these days.
Reading the monthly digests in Underground News, the Jubilee does seem to have become a less regular feature. I certainly haven't read about a whole line suspension for awhile. It doesn't exactly instil confidence when there are still issues being reported with train and signalling communication - I just hope it's really not going to let LU down during the Olympics.