Weird for me to see so many HOFers in this thread. To me a HOFer card could never qualify as the most boring card of all time, even if he looks like he's falling asleep.
The 72 minor league award card is a great submission for this thread, but at those awards cards have some color to them and are visually pleasing. For me one of the most boring cards that pops into my mind is the 1972 high number Pat Corrales in action card.
Can't see his face and somehow someone at Topps decided a photo in which HIS GLOVE ISN'T EVEN ON qualified as an "action" shot.