It would almost certainly be Gee Atherton. To anyone doubting the cardio or endurance of a DH racer know this – Ben Reid showed up at an XC NPS a few years back and destroyed everyone. On his 4x bike.
Gee would be there or there abouts at DH, 4X, BMX and he was hotly tipped to win the red bull rampage before injury so he would even beat Elbry in the freeride stakes. All he would need would be extra cardio to be competitive in xc, cyclocross and road, the Athertons (along with many DH racers compete in road and xc in the off season). It's safe to assume he could turn his hand to trials too.
Chriss Akrigg is pretty awe inspiring for sure, but there is little evidence he is truly a world beater in any one category, much less all of them.