I voted for Ragnar, mostly due to my Scandinavian heritage, but also because I follow the old maxim of playing the map. I really don't think you can give yourself a better leader than Ragnar on a water heavy map, his UU and UB combine to make water warfare and ship based attacks incredibly fun.
This being said however, I enjoy ragnar even on mostly landlocked maps, as even though his UB will be useless, he still has a solid UU (maceman with swordsman's innate 10% city attack bonus, plus being able to attack over rivers is just icing on the cake) and fin is always a decent trait, perhaps not as good as Org, but I think more flexible. Org helps you as you claim cities and expand, but fin helps you constanly. For example lets say you get a bad start are are locked in by a pro neighbor and have no metal readily available, fin will help you if you choose a peace based strategy (for the time being).
As always, there will be certain map types where other leaders will do better, but I think Ragnar has a lot going for him overall for an agg leader and he is an absolute monster on a water heavy map.
Also, I generally play on fractal maps, and if I'm lucky I'll have my continent secured or close to it during the medieval age. One of my fav aspects of Ragnar is should I choose to go mess with another continent (either because they are threatening the lead or just to be a bastard) all those zerkers I have laying around from initial conquests keep their amphibious promos when upgraded. So be it rifles, inf or mechs whenever I choose to go pillaging or conquesting abroad grabbing that first foothold city is much easier, esp with his UB that allows for more movement and general sneakiness in landing your troops.
Finally, in my mind nothing is more satisfying than the picture in my mind of hordes of burly blonde men with axes running screaming over the walls of an enemy city. It must be a throwback from all of the wonderful games I had as the Danes in Medieval 2 Total War.
edit: The above is even more satisfying when its the walls of timbuktu to finally wipe Mansa off the face of the earth, who peace vassaled to shaka long ago and was fueling that bastards research while shaka was out gobbling up half the world as I struggled to consolidate my continent.
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