in your example - "She's been opening my letters" - you would need a bit more exposition in order to properly describe the situation as "taking the cake."
I think you have misunderstood me. In my first post I wrote of her opening my letters as: 'the most annoying, frustrating, stupid, irresponsible, etc thing she has done'. I would have thought that the four adjectives following my 'most', ie four superlatives, gave 'some basis of comparison'.
Nevertheless, there is a difference between describing something as generally rude or bad versus something that "takes the cake." When using that phrase, it usually requires some basis of comparison, i.e. "I knew she didn't trust me, but when she opened my letters that really took the cake!" If my husband opened my letters without my permission, I might describe his behavior with many rude adjectives, but I wouldn't use "takes the cake" unless I was relating it to a previous situation.
In a later post I said (underlining added): 'Well, for me, for anybody to open my letters at all is a superlative example of something that is bad or unbelievable'. It is using 'take the cake' that tells us this. We do not need further exposition.
