Pah, I must endeavour to carry more interesting stuff with my bike.
The best I've done were:
a) About 70kg of sealed lead-acid batteries, which fitted unremarkably in panniers, but were the subject of much disbelief when I got them to the local household waste site (which is, naturally, uphill from here).
b) A stepladder. Not sure if it counts because I discovered that, having strapped it to the bike, it prevented me from turning left, so ended up wheeling it most of the way. Unfortunately it was chucking it down with rain so it didn't occur to me to take a photo.