Surely "McDoggeral".

The things about him is that, as Hamish Henderson pointed out, it's not at all easy to parody him. Almost inescapably, parodists will occasionally hit the note that McGonagall invariably swerves to miss.

But, borrowed from that Hamish Henderson essay, here is a piece that does hit that note - and it's not a parody of McGonagall, since it was written when he was six. It's a parody of the tradition of Irish street balladry from which McGonagall sprang (and which he diverted into something else):

The good sense of the Roman Catholics caused them to change their intentions of annoying with their complaints, the royal ear;
And they propos'd having a public dinner to celebrate the coronation, at which was expected the very best of good cheer;
Then an unexpected proosal came from the Protestants to the Catholics so very distinct and clear.
That all hostilities and jealousies should cease; and both parties assemble at dinner without any doubt or fear
At the celebration, celebration, oh! the wonderful celebration!

(From Irish Varieties" printed 1836 by J.D Herbert. The subject was George IV's state visit to Ireland in 1821.)