I hate when people say 'whatever' in a rude way.
I'll nominate "As you know" as either a tie or close second.
It's literally telling someone you're about to share something they already know, and you know you're about to do it.
I like this, too many useless sayings
Keeps me up at night, man.
You know what I mean.
Nevertheless, that couldn't be true.
I'll raise you, ‘as per my previous email'
that one always gets me
I always thought the expression "How come?" was awkward until I looked up the probable origins like "How has it come to this?" Likewise, the lengthier version of "I could care less" makes more sense if fully expressed - - something like "I could care less, I suppose, but at the moment I don't see how that would be possible".