Which is interesting, as my experience is similar, and there is usually significant bunching of trains and (very) large gaps between the bunches even when things are 'fine'.
But it didn't seem to be that way 10 or 15 years ago, when I often needed to catch the Piccadilly line in a very time-critical schedule, and it didn't ever seem to let me down. At the time I often commented about how it didn't have the bunching issues often seen on other lines and was actually remarkably reliable.
Of course this is only anecdotal, but it is based on a pretty large number of journeys over the last 15 years or so. Things definitely seem to have significantly changed for the worse.
On the wider question, I agree there are good and bad things to be said about all lines. I suppose my least favourite is the Waterloo & City simply because it is back to never running at weekends
which is pretty much the only times when I'm in London, so it is fairly useless to me now. Which is a shame - I was one of the people who used it quite a bit when it had Saturday running.