Backpacks. Based on the previous axiom (content to container weight), I'd chosen a backpack instead of panniers as my primary container (picture below). A backpack (stripped of unnecessary straps and belts), together with a bungee cord or two, is lighter than any pannier(s) of the same capacity. For example, my 40 l backpack was 1120 g, and is now 820 g after I cut off the waist belt and removed few other appendicies. For comparison, if you take 2 Ortlieb 'Back Roller' panniers, the capacity is the same, 40 l, the weight is almost twice as much, 1630 g (data taken from Ortlieb website).
P.s. Nowdays I don't use backpacks - I found much lighter alternatives. See stuff-sack setup below.
Waterproof bag. An even lighter alternative to the backpack is a waterproof stuff-sack/bag. My waterproof bag is almost one half of the weight of the backpack of the came capacity (40 l). The bag has additional advantage of waterproofness, but, alas, is not appropriate for hiking and, also, items inside are not as quickly accessible. Nevertheless, for a pure cycling trip, it's ideal in my opinion.