Dumb bell rows.
I prefer and use dumbell rows as it tends to isolate the back, without too much interferance from the biceps and lower back.
Although the barbell will allow me to lift alot more, the weight is shared with other body parts such as the biceps to a significant degree. On the other hand a proper dumbell lift using the shoulders as a focus will exclude the use of the biceps to a significant degree and allow me to feel intense pain in the back/lat area. (How i love it).
Furthermore I work my lowerback heavy with my deadlifts and stiff leg dead lifts anyway. The dumbell row kicks butt.
As for the best exercise to improve the back, I have to say that it is weighted chin ups as it is a nasty and painful (wonderful) workout. It seems to recruit the whole back much more so than some silly lat pull down machine.