I was going to say zeiss conquest with the z800 reticle! probably in the 4.5-14x or 3.5-10x

Whatever I chose I would get a reticle with multipe aiming points.....ie the leupold B&C, or the nikon BDC, or zeiss Zreticle, the Z800 would be the best for the 300 ultra mag, not the z600