armed robbery would definitely be the least worst of the three

But you wouldn't describe it in those terms, would you? You'd use a term that could be qualified:

Armed robbery is the least heinous of the three.

Then there's Jazzo's question about "That's one of the worst books I've ever read." I reckon that's all right, because in that sentence, you're talking about a grouping; the book to which you refer is one of a group of the worst. Within the group of books which you consider to be the worst you've ever read, there must be one that is more, ahem, heinous than the rest. That would be the worst book you'd ever read, because it was worse than all the bad ones. It would also be one of the worst books you'd ever read.