Most likely the EF 70-300mm f/4-5.6L IS. I've moved beyond expanding my focal range (coverage for 16mm to 600mm, or 1200mm if you count extenders), faster apertures (have f/1.2), dedicated macro (100L and MP-E 65mm), tilt/shift (TS-E 24L II). Now it's mostly about having the right tool for the job. The 24-70 II and 70-200 II make a wonderful combination, but the 70-200 II is big and heavy (as is the 100-400L) and on trips I generally need longer than 200mm, need f/2.8 less at the long end, and don't have time to fiddle with TCs. I have the 28-300L, and IMO, its IQ is equivalent to the 24-105L through the respective ranges - very good, but not the best possible. Since getting the 24-70 II, the 24-105 sat untouched...until I picked it up today to sell it off (which I did). Having upped the IQ at the wide end of the range, I'm looking to do the same at the long end, albeit by trading the convenience of a one-lens solution. I'd also gain the benefit of f/2.8 in the standard range, vs. f/4-4.5 for a good chunk of that. The 24-70 II and 70-300 L together are only ~2 oz more than the 28-300L, and either will fit in a Lowepro Lens Exchange 100 AW (meaning camera and one lens on a Spider Holster on the right side of the belt, the Lens Exchange with the other lens on the left, and easy mid-air changes).
So, no pics to show - the 28-300L is serviceable, delivers very good IQ in a convenient package, just looking to trade a little of that convenience for better IQ.