The question of who exactly was the first King of England is a fascinating one, albeit one that we are never going to be able to answer definitively. What proportion of modern-day England do we think an Anglo-Saxon King had to have control over for us to call him the King of England? And for how long? And what level of power do we expect him to have exerted? It's all very confusing. And yes, these questions make my head spin as well.

I have seen references to the following kings as 'the first King of England':

Egbert of Wessex (king 802-839)
For a short time he was bretwalda, or overlord of all the kingdoms of England. He had defeated the Mercians in battle, taken control over the smaller southern English lands, and in 829 received tribute from Eanred of Northumbria. But in 830 Mercia regained its independence, and Egbert's rule was once more restricted in its scope. His control over the north of England was tenuous even at its zenith, so he was merely a putative 'first King of England'.

Alfred the Great of Wessex (king 871-899)
The argument for Alfred as the first King of England rests not on the extent of his realm, but on the fact that from now on there was only one Anglo-Saxon kingdom of note - the Kings of Wessex went on to gain the whole of England. But the existence of the Danelaw splitting the country in two disallows the case for Alfred.

Edward the Elder of wessex (king 899-924)
Edward's campaigns made great dents in the Danelaw, as far as the Humber. In 920 York, Bramburgh, Strathclyde and the Scots submitted to him. His military conquests undoubtedly helped to pave the way for a united England - but still perhaps not enough for him to win the prize.

Athelstan of Wessex (king 924-939)
One of the more popular candidates, his victory at Brunanburh certainly wins him fame down the centuries. This great victory over the combined forces of his enemies. There is no doubt that, for a time at least, the writ of Athelstan held sway over a greater area in Britain than any other since the departure of the Romans.

Eadred of Wessex (king 924-955)
Eadred had to regain Northumbria not once but twice, as first Eric Bloodaxe and then Olaf Sihtricson came to claim a slice for themselves. After Olaf's expulsion in 954, Eadred could at last claim to be the King of England, although control was still shaky.

Edgar the Peacable (king of England 959-975)
King of Mercia and Northumbria after rebelling against his brother Eadwig, he inherited Wessex on his brother's death. The ruler of England, he reformed the coinage system and (probably) established the division of the Shires in the form that they were known until 1974. A worthy contender.

So, who wins? At the risk of sounding terribly dull and indecisive, it depends. But... Athelstan exerted his power for some years over most of what we now know as England and, although the north of England was unruly for some time afterwards, if we allowed that to disqualify him as King of England then kings centuries later would be in trouble in our definitions! So Athelstan pips it but all the above played some real part in the unification and rule of England.