Erm... I hate to pick sides in such an amazing matchup but... My money is on flandre. Yeah buu is superawesome powerful but Flandre's powers work on absolutes. You cant really dodge/block/deflect the absolute of destruction. The only way buu would win is if he went and vaporized all of flandre before she got a chance to act (and he is totally capable of this) however knowing buu he would want to "play" first...