Hmm, mega has two branches of motors, dunno if you want to include that but there is:

MEGA ZX
MEGA MF

MF would be their novarossi line of motors
ZX is their picco based stuff

Sooooo, the "best" MF engine would be: Mega MF12 3P MasMo Cera IFMAR
and the "best" ZX line engine would have to be: Mega ZX12 DSII Legal Race

But its up to you if you want to make Mega all just one line.