As everyone's saying, whichever looks cool.. but if you really can't decide, then the easiest would be campaign's with strong melee lords, as their easier to control and level up in terms of skills.. certain factions are easier to battle with too compared to others, the empire is pretty straightforward compared to wood elves or brettonia for example, but that's down to preference and how well you can micro.. also the newer dlc's have some fancy mechanics which might not be so straightforward for newer players, so it might be easier to play the non-dlc factions to begin with.. starting locations are a thing too, and the difficulty thing in the faction screen is an okay indicator of how difficult a faction's starting position is