The worst I've actually owned is also strangely the best: a belt drive Kenmore. Important note, before the Kenmore fans come after me, ready to beat me repeatedly with a Roto-Swirl agitator, it was the "worst" by default--I have only owned one, maybe two washers, both BD Kenmores.
If only that machine were the worst I've ever used... [Lord Kenmore sighs an unhappy sigh.]
The worst I've had to use in a rental arrangement is almost a cliché: early 1990s Frigidaire. No real reliability problems, except one needed a helping hand towards the end to get into spin. This surprises me, given the reputation of the WCI design. But neither machine did a very good job of washing clothes. I got by OK, mainly with a mix of care, plus the fact that most of my laundry needs are freshening, not dealing with horror loads. If I were the spouse of an auto mechanic, who insists getting work clothes as clean as possible, and insists that the Frigidaire will have to be the washer we use as long as it lasts, I'd probably have blown my brains out in a week.
Worst dryer: ca. 1990 Hotpoint, which took forever to dry a load of clothes. Of course, it may have been some issue with aging in that dryer.