Obviously I like them both, but for me, Conquistador is best.
The reason is the exploration. In Viking, you had to unlock areas and visit them as they became plot relevant, and as a result you mostly do the areas when there is something in the mainq uest thar requires you there. In contrast, in Conquistador the world felt like your oyster. You could go anywhere you wanted anytime just for exploring, and there were huge areas to the west that you never visited if you only follow the main plot. You could go into the unknown any time you wanted and truly felt like an explorer. IN Viking you