Ivar the Boneless, Svend Tveskæg ( Swein forkbeard), Canute the Great or William- in that order- the rest are just a bunch of wankers and wannabees
So, unless these were time travelling reindeer shaggers, they're not eligible since Britain didn't exist when Cnut was carving out the Danelaw. Besides, if we're going with anyone who ever invaded the British Isles, we'd just start and end with Caesar.
That wasn't the question though. Besides, William's invasion was hugely helped by the fact that the English army had to have rushed back to the South after having defeated Harald Hardrada in what they thought was the main theater of war (at the battle of......Stamford Bridge, of course). William swooped in like a carrion bird, and while he certainly obliterated the north of England in the succeeding decades (where the real opposition to him was based), he's basically notable as a military commander for one battle. I could easily argue as far as wars of conquest, Agricola's was more meaningful, or Plautius's.
Of course Harold had issues with Hardrada and Tostig and forced marches (Harold should have rested and adopted scorched earth and waited for William to approach him and extend his supply lines whilst Harold gathered reinforcements), but Caesars military also had distinct advantages when fighting here. Caesars was a glorified raid, Williams was a conquest. William of Orange was also a pretty important conquest if we are looking at later periods. Big implications in terms of religion and the constitutional balance between king and parliament. I will read up on Agricola and Plautius.
I'm not arguing Caesar's conquest was significant in any way - it wasn't. But he was the best commander faced by the English, which was the question. As far as Harald, in a feudal society it's very difficult to adopt scorched earth tactics in your own territory. Your vassals tend to really, really dislike that.
I should have been clearer; Caesar's two invasions did have consequences - they basically introduced England to the Mediterranean world. Without them, Claudius may never have invaded England at all, and much of England's medieval infrastructure was based on its Roman precedent (even if London isn't quite a direct descendant of Londinium). However, in the short run, yes, it was of very limited import, other than to convince the Gauls in Gaul that Rome was willing to go to great lengths to deal with Gallic allies/druids.
The Danelaw was carved out long before Cnut and was a much smaller territory. Svend and Cnut was the just the climax of the Danish viking invasions. And reindeers do not live in Denmark. You will have to go much further North in Scandinavia to find them but if post 17th Century is the standard i would vote for Gandhi. Defeating the British without firing a shot.