Well my least favourite key was going to be the microsoft windows start bar key thingy. Back before games would lock it out to stop you from accidently pressing mid fight I developed a deep hatred for it. The insert key annoys me from time to time when it accidently gets pressed and when I edit some writing it overwrites rather then adds.
But now that you mention it, a New Zealand key would make my least fav.
My favourite would be the treble clef. So sexy.
And Scroll Lock is used mainly on forms and data entry into spreadsheets and the like. What it does is switch the arrow keys from moving the text input bar thing, changing it to moving the focus between fields.
Open up excel and give it a go.
Tilde is used in programming from what I remember, been a while. Also can be used to add a bit of emphasis to a certain word to make it drag on and out I believe. Like helloooooooo could be changed to hello~ and it represents the dragging of the word and fades out...but all that part is just a guess

Gogo wiki.