Who has the highest official Dragon Ball Z power level?
I'm not sure how credible you consider the Dragon Ball Wiki, but they have compiled a list of power levels of most characters mentioned throughout the series, including GT.

The highest power levels in that list would be the old Supreme Kai's at at most 120 billion, while the next largest power level would be Gogeta SSJ4 at 25 billion.

See here for a more exhaustive list.