Genetics will determine the hardest (and simplest) body part to develop. Different for each of us.
For me and others, the best single machine workout was with the original Nautilus Pullover machine. The one with the pull down bar overhead and in front, which was used immediately (curl grip) after the set of pullovers. Now even some physical rehab centers are equipped with this, including other Nautilus type devices. By far the best pump and sense of fatigue that the back was worked to it's fullest and beyond. After all the sets were done, we moved to doing BB shrugs for the 12 to 15 reps, or so. Couldn't make a Tri set out of that because you were squeezed and strapped into the pullover machine. For me, too hard to get in and out of it fast enough.
One of the reasons why most guy's don't always get a great back pump is because the arms/biceps take the majority of the workload with most pulling movements and can't work the extremely powerful back/lats any where near their full potential. The Nautilus machine had the much stronger leverage on the elbows moving the weight, rather than the biceps. Most fine that a curl grip, in any row/chin/pulldown motion, will have better affect on the lat's/back, which allows the elbows to interact more.
There are elbow straps that men use when doing sitting cable pulls, giving greater focus on the back it's self. Also applied to overhead pulldowns and chins, though not maybe not the best choice..very hard to get use to in any overhead work. Elbows tend to not stay in the webbing of the straps. Might be worth a try though. Can always do ab/knee raises.
Would suggest as first choice (if not lucky enough to have pullover type machine laying around) BB/DB pullovers, Prefer the bent arm version over the straight, but that is up to you. Also chins (curl grip) or a pulldown bar. Doing pullover SS's with chins, row, etc always seems to be productive for most.
Another good option is using the BB row, with a set of extra wide grip, pulling bar inline with the shoulders (as much as possible)and to the upper chest. Than a set of medium grip BB rows to the lower rib box/upper abs. the last BB row set is using a medium to close curl grip, pulling to the lower abs. This is one tri set, with out stopping between each set. Max would be 2 tri sets. Good Luck.