Banquo. Well, the ancestor of kings would be more accurate. Fleance does not become king himself, but his descendants supposedly do a couple of hundred years later. None of this is historically accurate you realize.
Macbeth
The witches' predictions that he will be the father of a line of kings.
Banquo's heirs are prophesied to become kings. Macbeth tries to frustrate this by having Banquo and his child killed, but he fails because Fleance lives on to become the father of a line of kings
In Macbeth, the witches predict that Banquo's descendants will be kings, even though Banquo himself will not be. This prophecy ultimately leads to Macbeth's downfall as he becomes obsessed with eliminating any threats to his own position as king.
The wrong people. In order to thwart the prophecy about Banquo's children becoming kings, he has Banquo killed, but not his son Fleance. Fleance will go on to become the ancestor of kings. Macbeth killed the father when he should have killed the son. Then, in order to try to thwart the prophecy about Macduff, he has Macduff's son (and wife and other children) murdered. Macduff will go on to defeat Macbeth. Macbeth killed the son when he should have killed the father.
Macbeth is talking about the witches' prophecy to Banquo "Thou shalt get kings thou thou be none". From this Macbeth infers (although the witches did not actually say so) that his children would not be kings. Macbeth translates "thou shalt get kings" into Banquo being the father to "a line of kings". Well, the witches only said "kings"--they didn't say it was more than two. Then he imagines that none of his children would be kings with a couple of synechdoches: he describes the crown as fruitless (without children) and the sceptre as barren (incapable of having children) when it is the king, himself, which he imagines as fruitless and barren. I cannot hear the phrase "a fruitless crown" without imagining Carmen Miranda. Sorry if I have now put this image in your head.
A big effect
Sinel is Macbeth's father
Macbeth: all hail Macbeth! hail to thee, thane of Glamis all hail Macbeth, hail to thee, thane of Cawdor all hail Macbeth, that shalt be king hereafter Banquo: hail hail hail lesser than Macbeth, and greater not so happy, yet much happier thou shall get kings, though thou be none
The line of kings shown to Macbeth in his vision are intended to imply that Banquo's descendants will not only be kings but will be kings for ever. Naturally this was a polite flattery to King James, who was supposedly one of them. Anyway, Macbeth gets the point, although why it should distress him as much as it does is puzzling.
angus is one of the kings thanes
Drugged them.