Tough call. Only Out and backs Ive really been on havent been exactly complex. Theres Magnum, with the pretzel turn - fun but not crazy about it.
Then there is Shivering Timbers which has an awesome swoop turn. Villain, same thing only a bit more intense (though it is a double out and back). Then I have Nitro which hammerhead is a very good element.
Its probably a tie for Nitros Hammerhead and ST's swoop turn. Im just a sucker for the swoop turns mostly.