Of course this list is biased towards players that were not #1 for a long time themselves.
Becker was only #1 for 12 weeks given him a lot of matches against reigning #1's. Nadal is very impressive at 13 already, but will stagnate now he could be #1 himself for a long time. Lendl may be the most impressive in this list since for over 5 years during his prime he couldn't face the #1, certainly more impressive than Becker. Federer has a serious problem for this statistic, during his prime either he or Nadal was #1 which doesn't help his count. Murray could follow Becker's path here.
It would be nice to have a list of the winning % against reigning #1's.