Ted Kid Lewis, Owen Moran or Jack Kid Berg I would lean towards Moran or Berg as having the best stand out win, maybe Berg (Canzoneri or Chocolate). Crossing the ocean to take out Battling Nelson is no mean feat though
Jim Driscoll was unchallenged as the best pure boxer by Britishers themselves well up into the 1920s. I don't know when or he has been surpassed since. His four books on boxing, all of which I have read at one time or another since they are in my book collection, still make for good reading.