Well, I know the music is the priority, but i think also if you only in Ibiza for a holiday you should be thinking along the lines of which one is going to give you the best Ibiza experiance.
If you had asked me this 5 years ago...it would have been without a doubt We Love. Out of the 3 you suggested, I still say now " We Love" , but I must say recently it is losing a bit of it's magic. Still good though.
If you are going to do just one Club night, then I would go to Ushaiia on Sunday night then to Cocoon on the Monday (Especially if now it will open at 6pm).
Don^t know why I am typing in italics....it just decided to do it...it^s Swiss.