I think Harold might replace Alexander the Great as my new favorite general and ruler. The guy was a Viking king of Norway, but before that, he was the leader of the Varangian Guard of the Byzantine Empire. The Guard consisted of Norse/Vikings because some guy married the Emperor's sister and I guess he was a Viking or something.

Anyway, Harold was tasked with taking Sicily on his own, and he was great at it. For the first major city, he and his men dug under the walls and attacked when the enemy was eating dinner.
For the second major city, he pretended to have died. His men wanted him buried in the city, and the defending soldiers thought it would be an honor to have such a leader buried in their city, so they opened the doors. When they did, Harold jumped out and the Guard took the city.
The third major city is where he gains my favoritism. He noticed that the roofs of the houses were made of straw and such, and that birds nested in them. So he ordered his men to catch as many birds as they could. They smothered tar onto the birds and lit them on fire. When they were lit, they flew back to their nests and effectively caught the whole city ablaze. The Guard took this city without having to fight.

After his excursions in the Guard, he went on to claim his throne in Norway and attacked the British Isles in 1066, where he was fatally wounded.

But damn, fire birds. They need to make this a special unit in Civilization. Have it do as much damage as a nuclear missile or something.

My pick for best Scandinavian general goes to Gustav. Sorry fam but he's just A Tier.

For those who like Alexandros the Great, Kurosh and Darayavahush the Great did like things but were far better at it (most estimates put Iran at the end of Kurosh's rule as the biggest empire ever, proportionally by population.), and far for their time, too.