Mine is Henry VIII, as his impact on the monarchy was huge. He founded the Anglican Church and his reign is fascinating, the most interesting. He was both a ruthless tyrant and a loving family man. After the Tudors, the monarchy gets sooo boring (though I am quite fond of Charles II as I'm studying him for A level history).
He ended the Wars of the Roses by defeating Richard III and then maintaining his position in the face of two subsequent military challenges.
Married Elizabeth of York, uniting the houses of York and Lancaster, in a brilliant political move and, despite the political utility of his marriage, reportedly never even looked at another woman, a phenomenally rare thing in a medieval king, English or otherwise.
He rebuilt the power of the monarchy, and left the treasury in the black and considerably fuller than when he started, again a phenomenally rare thing in a medieval king, most of whom left the Crown in debt.
He managed to arrange excellent marriages for his children so that not only was his surving son king of England, but his eldest daughter was Queen of Scotland and his youngest was Queen of Spain (for a while).
He is also a direct ancestor of the current monarch, unlike some of the more famous ones whose lines were cut off.