That would be Kenneth mac Alpin. First to rule something like a united Scotland, even if a lot of his subjects might not necessarily have been so up on the idea.

If you are interested in historical Scottish politics the Declaration of Arbroath is probably far more interesting. You can just about read bits of it with modern eyes; but translated is better.

But it's not like we celebrate it or anything. There's plenty of opportunities to get drunk and sing songs already (Burns Night, St Andrew - and Scotland has taken New Year and given the world back a Hogmanay to be proud of).