Largely, but William was also the grandson of Charles I (and therefore nephew of James II) and was fourth in line of succession in his own right (after Mary, Anne, and the new baby).
She was actually not even on the list for most of her pre- reign having been declared a bastard by Henry VIII and Mary I.
Is there a place where you can see the historic lines of succession? For example, what was the line of succession when Victoria ascended to the throne, or when George III died?
No, if William has a son, that child will be third in line–Charles, then William, then William Jr. Harry would be bumped out of #3 to #4, and will only become monarch if all of William’s children (and their children) die before him. Or Parliament passes a law that says Harry is the monarch.
He would be third in line. Henry would be pushed to fourth.
I’d say that Queen Victoria has a decent chance at being the monarch who was (at some point in her life) lowest in the order of succession who eventually became the reigning monarch. Her grandparents - George III and Charlotte - had about 15 children, most of whom lived well into adulthood (ripe old ages for many, in fact,) but most of George III’s grandchildren were illegitimate or barred from the succession by virtue of their other parent’s Catholicism.
Otherwise, if there’s a clear-cut winner in this derby, the other frontrunners seem to me Henry VII or maybe William III. Or perhaps Lady Jane Grey, whom Edward VI had named his successor, but who was never crowned. Hard to say…
That’s how I read it too, as discounting the reply about Victoria.
Barring taking the crown by force, I think the record goes to George I.
Ah yes! Considering that the Act of Settlement was passed during George I’s lifetime, he would have jumped more than 50 spots to number one. I think we have a winner…
Ah, well let’s strike him from the list then. Henry VII seems a good bet if we’re counting those that seized the throne but at least had a nominal claim. William the Conqueror had a pretty dodgy claim as well, but that’s going waaaay back.
George I is certainly a good bet for non-military takeovers.
Depends on the criteria. I think he’s the winner, if the evident impossibility of having Catholics on the throne in his day doesn’t disqualify his 50-spot score from the game. Otherwise, I’d go with Henry VII.
Charley was correct. I responded to a post and failed to quote it. My mistake.
What about William I (The Conqueror)? Arguably, he was never on the list at all (depending on what Edward the Confessor had really promised).
You know, for someone nicknamed “the Confessor”, you would think we would have a better idea of what he actually got up to! 
ETA: Darn, pipped at the post again!
If that’s up for grabs, I’ll nominate king Canute. 
He had exactly zero legitimate claim to the throne …
He had a claim in his own right, just not a very good one.
Well, lessee, including in line those who might have plausibly survived…
William the Bastard - Arguably no legitimate claim, but Edward the Confessor did give him half-assed promises ( as he did others ). Best to leave it as “indeterminate” as it is hard to say who should come #2 after Edgar Atheling ( who should have been the unarguable #1, as the last of the direct line of the House of Cerdic ).
William II - #3. His elder brother Robert was skipped over and his other elder brother Richard died as a teen from a hunting accident.
Henry I - #4. After the above Robert, Richard and William.
Stephen - #7. Henry I had two legitimate children, William and Mathilda. William drowned as a teen and I’m giving Mathilda the #3 slot as Henry declared her the lawful heir. Henry’s older brother Robert ( passed over as above ) had a son, William Clito, who would have had been the likely #2. Stephen, Henry’s nephew, then had three older brothers - William of Sully ( passed over, considered unfit ), Theobald of Blois ( got the lion’s share of the family lands and did not have the ties in England that Stephen did ) and Odo, died as a teen.
Henry II - #1 or #3. Depends how you look at it. He was either heir #1 as son of Mathilda or behind Stephen’s two surviving sons ( I won’t count Baldwin, who probably died by the time Henry was born ).
Richard I - #2. After his father’s eldest, Henry the Young King. William, the eldest, died before he was born.
John - #4 or #5. After his elder brothers Henry, Richard and Geoffrey. Arguably after Geoffrey’s son, Arthur. Granted by the time Arthur was born a couple of his brothers were dead, so maybe #4 was the farthest he was ever back at any given moment in time.
Henry III - #1.
Edward I - #1.
Edward II - #1. Edward II had a whole slew of elder siblings, but all the boys ( John, Henry, Alphonso ) died before he was born.
Edward III - #1.
Richard II - #2 or #3. Probably #2 as his father ( who never reached the throne ) shouldn’t be counted, as he would have eventually followed him. At birth his father Edward, the Black Prince was #1 and his elder brother Edward of Angouleme was #2.
Henry IV - Let’s say #6. At the time of his birth his elder brothers had either predeceased him or died very shortly thereafter. But the three above were #1-3. His uncle Lionel of Clarence was #4 and his father John of Gaunt would have been #5.
Henry V - #1.
Henry VI - #1.
Edward IV - Ugh. Complicated. In fact I’ll stop here and let someone else work out the rest.
I propose we just skip the War of the Roses monarchs since legal succession was largely ignored in favor of ascension by arms.
I’ll take us through the Tudors:
Henry VIII - #2 after his brother Arthur
Edward V - #1
Mary I - Using the current rules (not those of the time which were really sketchy about even the possibility of a female monarch) she was #1 at birth and dropped down to #2 after the birth of her brother Edward V, and back up to #1 when Edward took the throne.
Elizabeth I was #2 at birth before being officially bastardized and the birth of Edward.
Actually there are two exceptions, the husbands of both Marys. William III was Mary II’s husband, but third in line in his own right. But Mary I’s husband was Philip II of Spain – though only consort in England, he was titled King because he was in fact King of Spain.
It’s also worth noting “King Henry the 2.5th” – Prince Henry the eldest son of Henry II was crowned King in his father’s lifetime, at the (elder) Henry’s behest. However, he died before his father, and Richard I was the successor to Henry II.
He wasn’t, at least to begin with. His father was still King of Spain, and he wouldn’t gain the throne of Spain until his father’s death in 1556. He was King of Jerusalem and King of Naples, though. He was titled King because Parliament titled him King. “Prince-Consort” was a title unknown in England until Parliament granted it to Prince Albert, over Victoria’s wishes that he be named King.