Lelouch at 0 difficulty. He could clear this fight with a look. Literally.
Then you look at the Code Geass mechs which get extremely overpowered before season 2 ends...I haven't even checked out the new stuff. With mechs: He clears most Naruto stuff up to war arc very easily. He clears all of One Piece. He clears most of Bleach up until the last two arcs fairly easily. He clears Shaman King up until the last 10 or so chapters.
Okay, for anyone thinking that the bijuu can break their jinchuriki out of Lelouch's geass, that loophole works on genjutsu. Genjutsu works by the user implanting their own chakra into the target's chakra stream and disrupting it to cause all the wacky things that they see/hear/ and feel.
Lelouch's geass works by tapping into the brain/mind directly and implanting a command which the victim is obligated to obey without question. Unless the bijuu can perform atomic level surgery and single-out the strand of grey matter within the brain that is currently holding the command or something, there is no way they will be able to do anything for their jinchuriki except to watch helplessly.
Now for this thread, Lelouch would just start with geass right off the bat and command gaara to kill himself, which he will happily do for the great and might Lelouch Vi Britannia