The Vikings only have 23 ships, most of which are slow and expensive. They are much better at fighting than getting gold, in part because of the Longship keyword. However, they don't have anything bigger than 3 masts, and it just so happens that their 3 masted ships are also the ones with the best combat abilities and cannon ranks. Therefore, as long as you pick a 3 masted Viking longship as your Viking gunship, you'll do alright. In addition, all of their common 3 masted longships are relatively similar and therefore this list doesn't really mean a whole lot - reverse the order and it could be another player's opinion.
Top 5 Viking Gunships:
With somewhat above-average speed for a Viking ship and with a very helpful ability, the Muninn is one of the best ships in their fleet. L-range guns are a help since the longships don't have the best firing arcs and you have to shoot over the relatively bulky oars and width of the ship in the first place.
This one is nice because she's cheaper than the others that cost the same even after adding a captain to the Huginn. Her guns are all different and her ability is very nice to have.
I wanted to rank this ship #1 because of her perfect cannons, but the overall package is just slightly less than what you get with the above two ships. That being said the Naegling can be the most destructive longship in the game, especially if you manage to use her ability (possibly by teaming her up with one or more of these 5 ships).
The Hrunting is in between the other 3 masters in numerous ways, which actually conspires to hurt her position on this list. She's expensive but not as much as the Naegling, but her ability isn't worth anything for a gunship, especially since you'll only have one cargo space available after adding a captain and helmsman. Her guns are accurate but only at short range, meaning you'll have to get extremely close alongside an opposing ship to have all three in range at once.
The Kalmar is an LE with just S speed, but she compensates with fantastic cannons and 5 cargo spaces, enough to add the necessary crew and still have room to use her ability.
Honorable mentions (in no particular order): Asgard