Obviously humour is very subjective, but I would say there's not a single dud episode amongst the first 8 seasons, where as now there are one or two episodes that I've seen that I've actually turned the TV off as I found them so un - funny. Having said that there are some later episodes which are very funny in parts. I thought the Simpsons movie was pretty good, although not as good overall as some of the early classics.
Whether quality has varied that much since season 8 I would find it difficult to judge as the episodes shown on British TV are not shown in any particular order, so never quite sure which series I'm watching