The 600mm f/4L IS II....and not just because I own one. The 24-70 II is certainly a great lens, but IMO it's what the original 24-70 should have been in the first place. All of the new superteles saw optical improvements, but the greatest IQ delta was with the 600 II, and while the 300/2.8 and 500/4 saw modest weight reductions, the 400 II and 600 II are 28% lighter than their predecessors. So...biggest IQ gain and tied for most weight loss is why I picked the 600 II.