I just saw a picture of some random poster's UI, and he had the default attack button in the first slot. Which always confuses me. Very early into my WoW career (I say career because I spent way too much time playing it, and wish I got paid for it) I started taking out that button and putting in, for the most part, my most used skills.
Death Knight: Death Grip. I had it in slot 1 when I was leveling and it just stuck. Yes, I've accidentally hit it while raiding, and that's why I move it to the slot right above it whenever in raids, but keep it in for dungeons in case I have to step up and oh shi- tank.
Druid: Feral Form, then Mangle when in Feral Form.
Rogue: Mutilate. Pretty self-explanatory.
Mage: Arcane Blast. See above.
Shaman: Lightning Bolt. ^
Hunter: Steady Shot. ^^
Paladin: Hammer of the Righteous. ^^^
Priest: Penance. ^^^^
Warrior: Mortal Strike. ^^^^^
Warlock: Incinerate. ^^^^^^
Monk: Jab. <(^.^<) (sorry, had to do it)
So... what do you guys do with it?